Do you really care about the format being PDF or is it about the books being FREE? I'd like to make common queries like yours easier. LearnAwesome is open-source, so of course you're free to contribute: https://github.com/learn-awesome/learn
To paraphrase a typical rant: “Making knowledge free and accessible is useless unless a libre format is used like Markdown.” Lol
The PDF beef is funnier in that people usually don’t know why they are morally opposed to PDF, it usually boils down to not liking Adobe Acrobat a decade ago, not accepting that page format preservation is a thing, or some beef with zooming or something. End of the day, it seeks to be digital paper.
End of the day, it’s an open standard with multiple implementations. Just by virtue of the US Federal Courts using it for millions of documents it will be usable for the foreseeable future, well beyond our lifetimes. (And there are many similar or even bigger examples)
Go to John Baez's awesome list "How to Learn Math and Physics":
https://math.ucr.edu/home/baez/books.html
And then to Library Genesis.
Kenneth Kuttler's books doesn't seem to be there though.
http://www.theassayer.org/cgi-bin/asbrowsesubject.cgi?class=...