PDF forgery is striking because it’d be so easy to do: find a useful research paper, edit it in any of the many PDF utilities, upload anywhere, wait for people to copy it (as they do), then take down yours; now you have an authoritative peer-reviewed research paper floating around the Internet with no links to you, in the perfect crime. (Or better yet: upload it directly to Libgen/Sci-Hub and let everyone else redistribute it.)

Seen as (unsigned) PDFs are basically images of documents, it's wild to claim that there are no forgeries, anyone doing a good ole fake document, fake diploma, fake wire transfer, fake articles of incorporation, etc... is using a pdf for faking. A nigerian prince scam sending an attachment mentioning 15.000.000$ locked up in inheritance would constitute a PDF forgery.

Not quite the scenario described (it wasn't an academic paper), but https://mjg59.dreamwidth.org/73317.html is my experience of dealing with a forged PDF. Part of the reason it's rare is that it's actually very hard to do it well!

Offtopic but the damage caused to an entire generation by Gwern's nicotine article is truly astounding. Obviously he's not responsible for the actions of others and it was written with good intentions but wow.