How to embed label in PDF?

Question

PDF

Greetings!
Let's say I have a PDF file that I want to distribute to a few people, but it must not get past their hands.
I want to add some information to the PDF code that will allow me to find out the source of the leak.
I opened the file with a text editor for editing, I saw places for comments there, at the end of each block. But I would like to know how to do it better.
Among the recipients there are people who are not far from IT topics, so if there are any standard places and tags for internal comments - tell me, well, where can I add non-standard too, where they will not be searched - but what will not affect the display of the document.
It would be nice to be able to do this automatically. By adding code to some specific place, so if there are any permanent rules for the location of blocks with service information, I will also be grateful for a hint.

Answer 1

www.pdfill.com/pdf_tools_free.html , specifically - first www.pdfill.com/pdftool_information.html then www.pdfill.com/pdftool_encrypt.html
Similarly, https://pdfbox.apache.org/commandline/ and pdftk