you mean sites like libgen.rs and b-ok.cc ?

I should also have mentioned this completely free book by richard hammack as an alternative: https://www.people.vcu.edu/~rhammack/BookOfProof/

Maybe, but this is not so bad, if you realize that the time and the effort you would spend on learning the material from such textbook is in some ways incomparable with the price of a couple of dinners at a restaurant.

Thing is that I just can't read PDFs.

If you have the means, try printing them little by little: chapter by chapter plus any back-of-the-book material. This way you can focus on one thing at a time, tactile paper in hand, without expending money on topics you don't yet need. You can annotate it, solve problems in the margins, file it, and reserve it for reference if/when you move on to successive chapters.

Sigh. I miss having a printer.

Try borrowing a good e-ink reader to see if it helps. If so, get a good one with larger screen if you'll be using more for textbooks.

Use an iPad, or any tablet, rather than smartphones.

I also read PDFs full-screen on my high-res, high-dpi laptop.

Try these two.

I have read tens of thousands of pages in PDF. (Yes, I checked)

Almost the same thing imo. It's a screen and I already sit so much in front of screens... all the more now that Elden Ring is out.