Add a text on an existing PDF document by appending something after the PDF content - pdf

Question

I would like to "overlay" a text onto an existing PDF document, by appending something at the end of the PDF file (after %%EOF). It is very important that nothing before the %%EOF is modified.
Is it even possible to do this ?
How can I "generate" what to append after %%EOF to do this, for a given text ? The technology doesn't really matter, once I have my "blob" I will just append it myself.
Thanks a lot!

Answer 1

How can I "generate" what to append after %%EOF to do this, for a given text ? The technology doesn't really matter, once I have my "blob" I will just append it myself.
That "blob" to append depends on the PDF to append it to. Essentially you'll have to parse the original PDF and find the page object for the page to overlay. Then you can append a new annotation or content stream with the overlay text, a copy of the page object with a reference to that new annotation or content stream, and a new cross reference section. In general you do that using a PDF library for your preferred programming language.
In a comment to your question you asked for example code to run and see the before/after and reverse-engineer it.
In the following example I use Java and the iText 7 PDF library (current development head but any 7.1.x version should do):
try ( PdfReader pdfReader = new PdfReader(SOURCE_PDF);
PdfWriter pdfWriter = new PdfWriter(TARGET_PDF);
PdfDocument pdfDocument = new PdfDocument(pdfReader, pdfWriter, new StampingProperties().useAppendMode());
Document document = new Document(pdfDocument)
) {
pdfWriter.setCompressionLevel(0);
Paragraph paragraph = new Paragraph("Hello! This text is added for Fratt");
paragraph
.setWidth(100)
.setBorder(new SolidBorder(new DeviceRgb(0f, 0f, 0.6f), 3))
.setRotationAngle(Math.PI / 4);
Rectangle box = pdfDocument.getFirstPage().getCropBox();
document.showTextAligned(paragraph,
(box.getLeft() + box.getRight()) / 2,
(box.getTop() + box.getBottom()) / 2,
1,
TextAlignment.CENTER,
VerticalAlignment.MIDDLE,
0);
}
(ShowTextAtPosition test testAddCenteredBorderedParagraph)
This adds the following rotated framed text to the first page of the source document:
In case of my example document the following "blob" is added after the original %%EOF:
16 0 obj
<</CreationDate(D:20060808104513+02'00')/Creator(TeX)/ModDate(D:20201221183247+01'00')/PTEX.Fullbanner(This is pdfeTeX, Version 3.141592-1.21a-2.2 (Web2C 7.5.4) kpathsea version 3.5.4)/Producer(pdfeTeX-1.21a; modified using iText® 7.1.14-SNAPSHOT ©2000-2020 iText Group NV \(AGPL-version\))>>
endobj
19 0 obj
<</BaseFont/Helvetica/Encoding/WinAnsiEncoding/Subtype/Type1/Type/Font>>
endobj
1 0 obj
<</Font<</F1 19 0 R/F73 6 0 R/F8 9 0 R>>/ProcSet[/PDF /Text]>>
endobj
2 0 obj
<</Contents[18 0 R 3 0 R 17 0 R]/MediaBox[0 0 595.2756 841.8898]/Parent 10 0 R/Resources 1 0 R/Type/Page>>
endobj
17 0 obj
<</Length 568>>stream
Q
q
0.70711 0.70711 -0.70711 0.70711 358.89 -66.87 cm
q
0 0 0.6 rg
251.62 401.57 m
351.62 401.57 l
354.62 404.57 l
248.62 404.57 l
251.62 401.57 l
f
Q
q
0 0 0.6 rg
351.62 401.57 m
351.62 374.93 l
354.62 371.93 l
354.62 404.57 l
351.62 401.57 l
f
Q
q
0 0 0.6 rg
351.62 374.93 m
251.62 374.93 l
248.62 371.93 l
354.62 371.93 l
351.62 374.93 l
f
Q
q
0 0 0.6 rg
251.62 374.93 m
251.62 401.57 l
248.62 404.57 l
248.62 371.93 l
251.62 374.93 l
f
Q
q
BT
/F1 12 Tf
255.94 391.23 Td
(Hello! This text is)Tj
( )Tj
ET
Q
q
BT
/F1 12 Tf
262.27 377.91 Td
(added for Fratt)Tj
ET
Q
Q
endstream
endobj
18 0 obj
<</Length 2>>stream
q
endstream
endobj
xref
1 2
0000009898 00000 n
0000009976 00000 n
16 4
0000009500 00000 n
0000010098 00000 n
0000010715 00000 n
0000009809 00000 n
trailer
<</ID [<98bc0d0e9347d0a066ab140ebd9ce62c><fa0dda3a13b826a6ecbd129bb048a3d0>]/Info 16 0 R/Prev 9003/Root 15 0 R/Size 20>>
%iText-7.1.14-SNAPSHOT
startxref
10764
%%EOF
Because of the pdfWriter.setCompressionLevel(0) in the code, the content stream is not compressed and you can read and understand it easily.