There's nothing wrong with epub, but there are lots of non-fiction books that have images, tables, and so on, I believe it's impossible to convert such books into epub or something like that. Also there are tons of books that were digitally published in pdf, and they also need to be pirated.
I don't think it's impossible to convert them to epub, iirc epub is actually Turing complete. But it doesn't make much sense to convert your carefully typeset/layed out PDF to a format whose entire point is that the typesetting and layout is dynamic and taken care of by your device.
Libgen, scihub, and z-lib are some of the most important projects on the net today. They're our library of alexandria, and western publishers are doing their best to burn them down.
Z-Library is definitely what you need, but there’s still no dedicated community on Lemmy that I know of. You’ll find all that you need to access it in Reddit (r/zlibrary). Anna’s Archive is also very good, but sometimes the latest releases aren’t found there, while they are on Z-Lib
Try Z-lib, you'll probably find what you're looking for there
Link for Z-Lib : https://singlelogin.re
Note: You need to create an account but that's pretty straightforward, if you run out of downloads you can use their telegram bot.