To be honest it's little hard to find textbooks in PDF version, but it's not impossible. You need to do little work. Here are few sites that i use to find pdf versions of books. Orelse use the google search and put "+ rapidshare" after name of the book, it will redirect to you another website where the books are stored. There are lots of books in rapidshare or you can try megaupload etc.

Good Luck. Here are few sites that might be helpful to you.

avaxhome.ws/

library.nu/