https://www.pdfdrive.com/

I'd recommend uploading any PDFs you download from z-library and the above site to VIRUS TOTAL:

https://www.virustotal.com/gui/

"free ebooks" are notorious for viruses like RAT.

Stay safe.

If you wish to convert PDFs to EPUB, there is a great bot on Telegram: https://t.me/newfileconverterbot