Many years ago I had a discussion with a friend about something similar, in the sense of tracking PDF downloads. He wanted to sell educational material in PDFs

Many years ago I had a discussion with a friend about something similar, in the sense of tracking PDF downloads. He wanted to sell educational material in PDFs and he wanted to be able to track pirated copies and who shared his paid PDF and that sort of thing. My proposal was something similar, but using steganography inside the PDF content instead of using metadata that can be (relatively easy) stripped. Each time someone bought the PDF, some trivial data (email, date, IP...) would be embedded in the first and last page of the PDF (title page & blank page) at specific coordinates with very small font size and almost white color (same as the page), so that later someone could read them if needed. Yeah, a silly solution and perhaps that's why it didn't move any further past the proposal phase. Hope I'm not giving any ideas to Elsevier and all the other greedy publishers with this ;-)