title_body: Can I use books from pdfdrive.com (legally) for my learning purpose?
upvoted_answer: First, in USA pdfdrive is probably operating on the fringe of legality. They scrape the internet for free pdf files to make it available in their service. I see several books there which shouldn't be there, which means that someone made an unauthorized copy and then put it on their website. Then PDFDRIVE will locate this copy and store it until someone complains. PDFDRIVE will remove things upon request, but of course, many publishers are too busy to complain. 
  
  The legal consequences may differ depending on what country you in. Generally speaking though, the consequences are more severe for people who share or distribute copyrighted material without permission. If you download it for private use, you are unlikely to pay a penalty. I am speaking generally. 
  
  I would think of it in another way. What is the right thing to do? Is the ebook easily available in your country? Is the cost of trying to obtain a legal copy significantly higher than other books produced in your country? 
  
  Also: what is your status/economic level? Generally if you are a student or teacher, you face fewer legal consequences (if any). If your monthly income is $500, and the cost to buy a foreign-produced book in your country is $75, I would consider that price to be excessive. Academic textbooks in particular are expensive for people in developing countries (and many of these publishers  try to discount their prices for customers in that region). 
  
  One way to deal with this ethically is to try first, buy later. If you find an unauthorized copy of an ebook and end up using it a lot, perhaps you will feel a duty to buy it later. 
  
  Finally, there's a lot of free and low cost ebooks (and wikipedia). While certain well-known books may cost a lot, often there are low-cost books which are equivalent or better. It just takes a little more time to find them.