I am writing an application that has to read and interpret data stored in some PDF files. The reading part is done but I am only able to get a dump of all the words on a page and not the format of the words. What I mean is that if I have to extract a table, I am getting the numbers in the table but not the markup which defines the table.
Further, there is some formatting used which displays a few of these numbers within parentheses (meaning that those numbers are negative) but the parentheses themselves are not part of the text. Hence, I am not able to distinguish between positive and negative numbers present in the PDF table!
How do you get the PDF markup along with the text? Is a PDF similar in structure to an XML with tags used to markup tables etc.? If not, then, is there a resource which describes the salient features of the PDF DOM?
I am using VBA and the Acrobat library (AcroExch etc.)
There is no such thing as "PDF markup" in the sense of HTML etc. A table in PDF cannot be distinguished from line art, other than by using OCR, which can be error-prone if the layout is complex. It is simply drawn using geometrical shapes, like in a vector-based graphics program.
"Is a PDF similar in structure to an XML with tags used to markup tables etc.?"
No, not at all.
And there is no such thing as a 'DOM' either. Google for a file named *PDF32000_2008.pdf*. The current PDF specification for v1.7 (ISO spec) is that file. You should be able to locate it on the Adobe website.
As omz stated, text inside PDF does not really have a structure. You can take a look on the specification here. However, for some very specific files, there is something called PDF Tags, or PDF Marked Content, which is fairly new, and it aims to give PDF documents some kind of structure. If you target this kind of files specifically, you might be able to achieve something. Take a look on chapter 10 (Document Interchange) of the Adobe's specification for further details.
Maybe what you want to achieve can be done with less effort and faster by using TET, the Text Extraction Toolkit made by the fine folks from pdflib.com ( http://www.pdflib.com/products/tet/ ) ??
AFAIR, the TET has some (limited) support for table detection as well....
Related
I want to generate a technical report from lisp (AllegroCL in my case) and I studied various packages/project to help me do this.
Requirements:
Need to generate a PDF
May create an intermediate format like RTF, Restructured TEXT, HTML, Word DOC or Latex
Need to be flexible to be able to add content throughout my application
Need to handle Multi-Page, Headers, Footers, Tables, inclusion of Images.
Possibilities:
cl-pdf and cl-typesetting: I checked this one out and it works for now, but is there a better alternative?
Some Latex generator, but ???
Question:
Do you know alternatives to easily generate (PDF) reports from lisp. What is the best workflow to go for?
we are using cl-pdf and cl-typesetting for the last 3 years and it has numerous issues... (like its confusion around encodings, or silently not rendering things that don't fit, or...) so, i don't recommend new development based on them.
currently we are in the process of moving all our export mechanisms to open document format. openoffice is all happy with it, and there's a plugin for ms office, too.
there's .fodt, the so called flat open document text format, which is a mere xml file describing a document. generating it is as easy as generating xml files.
you can also make parts of your document read-only with a password (insert a section and mark it read-only and protected by a password. when generating the xml, you can generate random hashes as password...).
Like the title says. Reason I ask is that we're converting PDFs to formatted ASCII text (using pdftotext) and only want to display the ones that look reasonably sane.
PPT files tend to have text over images, diagonal text and others things that don't translate to ASCII very well, so we'd like to filter them out if we can.
The creating application of a PDF is listed in its XMP metadata. You can see this quite easily in Acrobat 9 (and I believe earlier): go to File > Properties, click Additional Metadata..., then go to Advanced and it's listed under both XMP Core Properties and PDF Properties:
xmp:CreatorTool: Microsoft PowerPoint
pdf:Creator: Microsoft PowerPoint
I'm guessing you want to find this programatically, so you'll need to find a library to read this metadata that works with your language. Here is a list of some XMP tools.
Short answer:
No, I don't think so.
Long answer:
No, I don't think so, because there are may ways to convert a PowerPoint file to pdf, for example Adobe Acrobat and PDFCreator and many many others. It's up to the converters to embed specific information in the PDF file, even if you find a way to detect PowerPoint-source pdf from one convert, the same method may not work for another.
Even longer answer:
No, I don't think so, because of the reasons described in the "long answer". And I don't think detecting the source of the PDF is the best approach to the problem you are trying to solve. Not just PowerPoint produces overlapped text and images. I think it's much better to detect the actual layout of the PDF file. If there are overlay of image and text, then you do some filtering or pre-processing to cater for that.
Your reasoning is very arbitrary - there are surely plenty of PPT files without the features you describe, and plenty of PDF files with them, that were generated from another source.
In theory a better method would just be to detect when these "unwanted" situations occur. However, even though the PDF format is partly open (only for reading, apparently, so it's not truly an open format), extracting complex data like that would be incredibly difficult.
All PDFs can have this problem regardless of their source. Most desktop publishing suites are capable of outputting PDF and are often sold boasting their high quality and flashier PDF presentations ...
A "saner" method would be to use a PDF parser, ITextSharp, or pdfNet...etc, Using the library of your choice, find all image rectangles, and all text rectangles, SORT THE RECTANGLES, and then see if there is substantial overlap of text and image rects -- ignoring image to image overlaps. If so, reject the page and/or document.
That won't be perfect, but at least it's going to catch many PDFs that aren't sane, regardless of source. Other heuristics to add would include color analysis. (i.e. are the colors in the overlapping region sufficiently different to allow "sane" results?)
Best of luck to you
It might put its name in the creator or producer info, but I don't have a copy to check this theory with.
In general, it is not an easy task to programmatically determine (reliably) where a file came from or how it was generated based on its contents. After all, a file is just a collection of bits.
Unless you have a lot of resources to expend building the heuristics to determine whether a file looks "reasonably sane" according to your needs, I would consider this a task for human beings.
some converter from ppt to pdf preserve creator in comments at begin of pdf.
I think that PDF's generated from most applications seem to be the same. It may have some meta-data that you can read from the file...
My objective is to extract the text and images from a PDF file while parsing its structure. The scope for parsing the structure is not exhaustive; I only need to be able to identify headings and paragraphs.
I have tried a few of different things, but I did not get very far in any of them:
Convert PDF to text. It does not work for me as I lose images and the structure of the document.
Convert PDF to HTML. I found a few tools that helped me with this, and the best one so far is pdftohtml. The tool is really good presentation wise, but I haven't been able to successfully parse the HTML.
Convert PDF to XML. Same as above.
Anyone has any suggestions on how to tackle this problem?
There is essentially not an easy cut-and-paste solution because PDF isn't really very interested in structure. There are many other answers on this site that will tell you things in much more detail, but this one should give you the main points:
If identifying text structure in PDF documents is so difficult, how do PDF readers do it so well?
If you want to do this in PDF itself (where you would have the majority of control over the process), you'll have to loop over all text on pages and identify headers by looking at their text properties (fonts used, size relative to the other text on the page, etc...).
On top of that you'll also have to identify paragraphs by looking at the positioning of text fragments, white space on the page, closeness of certain letters, words and lines... PDF by itself doesn't even have a concept for a "word", let alone "lines" or "paragraphs".
To complicate things even more, the way text is drawn on the page (and thus the order in which it appears in the PDF file itself) doesn't even have to be the proper reading order (or what us humans would consider to be proper reading order).
PDF parsing for headers and its sub contents are really very difficult (It doesn't mean its impossible ) as PDF comes in various formats. But I recently encountered with tool named GROBID which can helps in this scenario. I know it's not perfect but if we provide proper training it can accomplish our goals.
Grobid available as a opensource on github.
https://github.com/kermitt2/grobid
You may do use the following approach like this with iTextSharp or other open source libraries:
Read PDF file with with iTextSharp or similar open source tools and collect all text objects into an array (or convert PDF to HTML using the tool like pdftohtml and then parse HTML)
Sort all text objects by coordinates so you will have them all together
Then iterate through objects and check the distance between them to see if 2 or more objects can be merged into one paragraph or not
Or you may use the commercial tool like ByteScout PDF Extractor SDK that is capable of doing exactly this:
extract text and images along with analyzing the layout of the text
XML or CSV where text objects are merged or splitted into paragraphs inside a virtual layout grid
access objects via special API that makes it possible to address each object via its "virtual" row and column index disregarding how it is stored inside the original PDF.
Disclaimer: I am affiliated with ByteScout
PDF files can be parsed with tabula-py, or tabula-java.
I made a full tutorial on how to use tabula-py on this article. You can tabula in a web-browser too as long as you have installed Java.
Unless its is Marked Content, PDF does not have a structure.... You have to 'guess' it which is what the various tools are doing. There is a good blog post explaining the issues at http://blog.idrsolutions.com/2010/09/the-easy-way-to-discover-if-a-pdf-file-contains-structured-content/
As mentioned in the answers above, PDF's aren't very easy to parse. However, if you have certain additional information regarding the text that you want to parse, you can pull it off.
If your headings are positioned at specific parts of the page, you can parse the PDF file and sort the parsed output by coordinates.
If you have prior knowledge of the spacing between headings and paragraphs, you could also leverage this information to parse the file.
PDFBox is a PDF parsing tool that you can use for extracting text and images on top of which you can define your custom rules for parsing.
However, for parsing PDFs you need to have some prior knowledge of the general format of the PDF file. You can check out the following blogpost Document parsing for more information regarding document parsing.
Disclaimer:I was involved in writing the blogpost.
iText api:
PdfReader pr=new PdfReader("C:\test.pdf");
References:
PDFReader
I'd like to write some (java) code that takes a PDF document, and creates named destinations from all of the bookmarks. I think the iText API is the easiest way of doing this, but I have never used the API before.
How would you go about writing this sort of code with the iText API? Can iText do the parsing needed to manipulate existing PDFs by itself? The kind of manipulations I am thinking of are:
Open,
Find bookmarks,
Create destinations,
Save,
Close.
Or is there a different API that would be better?
Followup: I submitted a patch to iText a few months ago (it has now been accepted and is part of HEAD) that adds text parsing capabilities to iText. PdfBox (mentioned below) has (had?) problems with reading newer PDFs that use xref streams instead of the older xref table format.
Another library that is very good at parsing existing PDF files is PdfBox It can also be used for modifying an existing PDF. FYI - this is the text parser that Lucene uses.
I will also mention that iText does have the ability to parse a PDF file, it's just not great at parsing the text content on each page. If you are looking at accessing the PDF higher level constructs (Dictionaries, etc...) that are used for storing bookmarks, etc... and you don't mind getting your hands a little dirty with reading the PDF spec, you can absolutely do what you are asking about (we do it quite a bit ourselves).
The PDF Spec is big, but readable for the most part, and you don't have to worry about the bulk of it (which is geared towards actual page content and rendering) if all you are trying to do is extract bookmarks.
I'll just warn you up front that you may be disappointed with this. iText isn't really intended to be used as a parser. It's really more for creating entirely new PDF documents, but you can take a whack at it.
To start, using iText, you won't be able to modify the existing PDF document. What you can do, though, is to make a copy with the additional features that you want. (If somebody else knows better, please let me know, this drives me crazy.)
What you will want to do is create a PdfReader object from an input stream on your source file. Then create a PdfCopy object (which is just an extended PdfWriter that makes getting data from an existing source more convenient) for your destination.
As far as I can tell, the bookmarks cannot be obtained from iText at all. Another library may be needed. I think jpedal may have the ability to extract them (it can get them as an XML document, which you may then have to parse to get what you want.) However you get them, you can then add them to a java.util.List, and set that list as outline on the PDFCopy. The bookmarks themselves are just HashMaps with a particular set of keys. I'm not sure what all of the values are, but they include "Title", "Action" (which seems to be where you'd specify that this is a named destination, though I don't know what that value would be), and "URI" (which is used if this is an external link -- I suspect that this would specify the name of the named destination that you're linking to). Again, the specifics are hard to find.
Then iterate over the pages of the reader, importing each page to the PdfCopy. this page may help you.
Sorry I'm not more helpful to you. Good luck.
P.S. If anybody else knows of a better tool that's either (L)GPL or BSD licensed, I'd love to hear about it.
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For a small project I have to parse pdf files and take a specific part of them (a simple chain of characters). I'd like to use python to do this and I've found several libraries that are capable of doing what I want in some ways.
But now after a few researches, I'm wondering what is the real structure of a pdf file, does anyone know if there is a spec or some explanations anywhere online? I've found a link on adobe but it seems that it's a dead link :(
Here is a link to Adobe's reference material
http://www.adobe.com/devnet/pdf/pdf_reference.html
You should know though that PDF is only about presentation, not structure. Parsing will not come easy.
I found the GNU Introduction to PDF to be helpful in understanding the structure. It includes an easily readable example PDF file that they describe in complete detail.
Other helpful links:
PDF Succinctly book is longer and has helpful pictures.
Introduction to the Insides of PDF is a presentation that isn't as in-depth but gives a quick overview and has lots of pictures.
When I first started working with PDF, I found the PDF reference very hard to navigate.
It might help you to know that the overview of the file structure is found in syntax, and what Adobe call the document structure is the object structure and not the file structure. That is also found in Syntax. The description of operators is hidden away in Appendix A - very useful for understanding what is happening in content streams. If you ever have the pain of working with colour spaces you will find that hidden in Graphics! Hopefully these pointers will help you find things more quickly than I did.
If you are using windows, pdftron CosEdit allows you to browse the object structure to understand it. There is a free demo available that allows you to examine the file but not save it.
Here's the raw reference of PDF 1.7, and here's an article describing the structure of a PDF file. If you use Vim, the pdftk plugin is a good way to explore the document in an ever-so-slightly less raw form, and the pdftk utility itself (and its GPL source) is a great way to tease documents apart.
I'm trying to do pretty much the same thing. The PDF reference is a very difficult document to read. This tutorial is a better start I think.
This may help shed a little light:
(from page 11 of PDF32000.book)
PDF syntax is best understood by considering it as four parts, as shown in Figure 1:
• Objects. A PDF document is a data structure composed from a small set of basic types of data objects.
Sub-clause 7.2, "Lexical Conventions," describes the character set used to write objects and other
syntactic elements. Sub-clause 7.3, "Objects," describes the syntax and essential properties of the objects.
Sub-clause 7.3.8, "Stream Objects," provides complete details of the most complex data type, the stream
object.
• File structure. The PDF file structure determines how objects are stored in a PDF file, how they are
accessed, and how they are updated. This structure is independent of the semantics of the objects. Sub-
clause 7.5, "File Structure," describes the file structure. Sub-clause 7.6, "Encryption," describes a file-level
mechanism for protecting a document’s contents from unauthorized access.
• Document structure. The PDF document structure specifies how the basic object types are used to
represent components of a PDF document: pages, fonts, annotations, and so forth. Sub-clause 7.7,
"Document Structure," describes the overall document structure; later clauses address the detailed
semantics of the components.
• Content streams. A PDF content stream contains a sequence of instructions describing the appearance of
a page or other graphical entity. These instructions, while also represented as objects, are conceptually
distinct from the objects that represent the document structure and are described separately. Sub-clause
7.8, "Content Streams and Resources," discusses PDF content streams and their associated resources.
Looks like navigating a PDF file will require a little more than a passing effort.
If You want to parse PDF using Python please have a look at PDFMINER. This is the best library to parse PDF files till date.
Didier have a tool to parse the PDF:
http://didierstevens.com/files/software/pdf-parser_V0_4_3.zip
or here:
http://blog.didierstevens.com/programs/pdf-tools/ which cataloged several related pdf-analysis tools.
Another tool is here:
http://mshahzadlatif.wordpress.com/2011/09/28/view-pdf-structure-using-adobe-acrobat-or-a-free-tool-called-pdfxplorer/
Extracting text from PDF is a hard problem because PDF has such a layout-oriented structure. You can see the docs and source code of my barely-successful attempt on CPAN (my implementation is in Perl). The PDF data structure is very cool and well designed, but it's easier to write than read.
One way to get some clues is to create a PDF file consisting of a blank page. I have CutePDF Writer on my computer, and made a blank Wordpad document of one page. Printed to a .pdf file, and then opened the .pdf file using Notepad.
Next, use a copy of this file and eliminate lines or blocks of text that might be of interest, then reload in Acrobat Reader. You'd be surprised at how little information is needed to make a working one-page PDF document.
I'm trying to make up a spreadsheet to create a PDF form from code.
You need the PDF Reference manual to start reading about the details and structure of PDF files. I suggest to start with version 1.7.
On windows I used a free tool PDF Analyzer to see the internal structure of PDF files.
This will help in your understanding when reading the reference manual.
(I'm affiliated with PDF Analyzer, no intention to promote)
To extract text from a PDF, try this on Linux, BSD, etc. machine or use Cygwin if on Windows:
pdfinfo -layout some_pdf_file.pdf
A plain text file named some_pdf_file.txt is created. The simpler the PDF file layout, the more straightforward the .txt file output will be.
Hexadecimal characters are frequently present in the .txt file output and will look strange in text editors. These hexadecimal characters usually represent curly single and double quotes, bullet points, hyphens, etc. in the PDF.
To see the context where the hexadecimal characters appear, run this grep command, and keep the original PDF handy to see what character the codes represent in the PDF:
grep -a --color=always "\\\\[0-9][0-9][0-9]" some_pdf_file.txt
This will provide a unique list of the different octal codes in the document:
grep -ao "\\\\[0-9][0-9][0-9]" some_pdf_file.txt|sort|uniq
To convert these hexadecimal characters to ASCII equivalents, a combination of grep, sed, and bc can be used, I'll post the procedure to do that soon.