If you're not sure of the legality of a site, check it out!
Question a professor forwarded to the library: A colleague of mine at [large international company] told me about libgen.org, where you can access pdfs of books free of charge. I found a PDF of [a book required for class] there. If you think it would be legal, I'm sending it to you so you can share it with the rest of the class.
Answer from the library: LibGenis a pirate site based in Russia and has been shut down more than once in the past, but continues to reappear. The copy of [the book required for class] is copyrighted, and the copy on libgen.org is not licensed. We recommend that you do not forward the PDF or the URL to your students since it was not uploaded legally. There are copies in the library as well many options for students to purchase the book in print or electronic format: Amazon, Barnes & Noble, U. Chicago Press, and iBooks.