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This question already has an answer here:

Is there a way to create a secure hidden watermark. The watermark packages do not seem secure, anyone can edit the pdf and change the text.

I guess it would convert the text in to an image and include that.

marked as duplicate by Peter Grill, siracusa, Stefan Pinnow, schtandard, Zarko Sep 18 at 7:11

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    One can set up/modify the PDF security parameters to prevent editing (with, for example, Adobe Acrobat). However, that has nothing to do with TeX. – Steven B. Segletes Sep 13 at 11:03
  • @StevenB.Segletes I believe pdf's are not secure, the security is easily broken due to a flaw... not sure if they have fixed it. In any case, creating an image would be best. Ideally something that can detect the water mark in some way but it is not visually viewable. – Chieron Sep 13 at 11:49
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    You can always add non-visible text to a document in any number of ways: 1) make it vanishingly tiny, 2) make it the background color, 3) include it as a graphic, 4) accsupp package approach, 5) introduction of non-random typos, 6) etc. You don't specify how it must be detectable, though (via software, or via inspection). – Steven B. Segletes Sep 13 at 11:54
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    Also, you have not indicated the purpose of the "watermark". Are you hoping to detect document tampering? Preserve proof of origination? – Steven B. Segletes Sep 13 at 12:07
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    Possible duplicate of How to add an invisible watermark to PDF documents?. Not sure what you mean by secure though. – Peter Grill Sep 13 at 14:28
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Have you tried to create a PDF/A document? Once created, it should not be editable.

Say that the file you create is watermarked-file.pdf

You can create the PDF/A version by means of another document:

\documentclass[a4paper]{article}
\usepackage{pdfpages}
\usepackage[a-1b]{pdfx}
\begin{document}
\includepdf[pages=-]{watermarked-file.pdf}
\end{document}

Or you can just include \usepackage[a-1b]{pdfx} in the preamble of your document, to avoid the additional step.

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