How to detect if a pdf is text searchable or non text searchable? - vb.net

Question

i have a set of pdfs, from which i want to process( VB.NET) only those which are non text searchable, can you please tell me how to go about this?

Answer 1

Generally speaking, the way to do this is open up each page and rip the content stream and see if any text operators are executed that place text on the page.
Let me explain what that means - PDF content is a small RPN language that contains operations that mark the page in some way. For example, you might see something like this:
BT 72 400 Td /F0 12 Tf (Throatwarbler Mangrove) Tj ET
Which means:
Begin a text area
Set the position of the text baseline to (72, 400) in PDF units
Set the font to a resource named F0 from the current page's font resource dictionary
Draw the text "Throatwarbler Mangrove"
End a text area
So you can try short cuts
does my page's resource dictionary contain any fonts?
This will fail in some cases because some PDF generation tools put fonts into the resource
dictionary and don't use them (false positive). It will also fail if the page content contains a Form XObject which contains text (false negative).
does my page's content stream have BT/ET opertors?
This will get you closer, but will fail if there is not content in them (false positive) or if they're not present, but there's a Form XObject which contains text (false negative).
So really, the thing to do is to execute the entire page's content stream, including recursing on all XObject to look for text operators.
Now, there's another approach that you can take using my Atalasoft's software (disclaimer, I work for Atalasoft and have written most of the PDF handling code, I also worked on Acrobat versions 1-4). Instead of asking, does this page contain any text, you can ask "does this page contain only a single image?"
bool allPagesImages = true;
using (Document doc = new Document(inputStream))
{
foreach (Page p in doc.Pages)
{
if (!p.SingleImageOnly)
{
allPagesImages = false;
break;
}
}
}
Which will leave allPagesImages with a pretty decent indication that each page is all images, which if you're looking to OCR is the non-searchable documents, is probably what you really want.
The down side is that this will be a very high price for a single predicate, but it also gets you a PDF rasterizer and the ability to extract the images directly out of the file.
Now, I have no doubt that a solid engineer could work their way through the PDF spec and write some code to extend iTextPdfSharp to do this task I think that if I sat down with it, I might be able to write that predicate in a few days, but I already know most of the PDF spec. So it might take you more like two weeks to a month. So your choice.

Answer 2

I think this option could be your consideration, though I haven't tested the code yet but I think it can be done by read the properties for each PDF files that you want to proceed.
You might check this link :
http://www.codeguru.com/columns/vb/manipulating-pdf-files-with-itextsharp-and-vb.net-2012.htm
You have to read the producer properties right after you proceeded it. That's just only example. But my advice please include your code here so we can give a try to help you. Bless you