I know PDF providers who visibly print the customer's name or number in the header of every page, along with short copyright text. I use qpdf --stream-decompress to make the PDF into human-readable PostScript, and then Python+regex to remove each header text, which stand out a bit from other PDF elements. The script throws an error if more or fewer elements than pages have been removed but that hasn't happened yet. Processed documents sometimes have screwed-up non-ASCII characters in the Table of Contents for some reason but I don't have the originas anymore so IDK if it's my fault. Still, I wouldn't share the PDFs unless in text-only or printed form because of any other steganographic shenanigans in the file. I would absolutely torrent them if I could repurchase them under a new identity and verify that the files are identical.

BTW, has anyone figured out how to embed Python code in PDF? The whitespace always gets reencoded as x-coordinates so copy&pasting it never preserves indentation. No, you can't use the Ogham Space Mark (Unicode's only non-blank character classified as a space) for indentation in Python, I tried.