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
Nerds love to bike shed crap like this and bite the hand that feeds.
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)
Not just that, but if you actually look at the implementation its pretty understandable. 90% of use cases are covered by the easiest parts to parse too. PDF might not be perfect, but honestly not much comes close.
Not sure if you needed PDF specifically, but this site has a lot of good OER textbooks, digitized nicely as HTML + MathJax: https://math.libretexts.org/Bookshelves
