Well, they are out of business for the time being. (Knowledge isn't free.)This is the best place I'm currently aware of for "bootleg" PDFs and eBooks. They have a very large library on many subjects. Mostly items originally from notable publishers. (For really obscure documents, a good option is archive.org.) I've been utilizing them since 2019 with no virus or any other problem.
You can download a certain number of titles for free daily. If you want more, then a donation will empower you for a month. I do believe you have to register to access the content they provide.
I am not at all averse to purchasing physical books, and don't even like reading a screen. But I don't really need or want a physical copy of everything that I might have a limited interest in, or want to consult for reference purposes. Also, as I listen to a lot of audiobooks, I often like to get an e-copy to go along with them, to check spelling, review details, etc.