Project Gutenberg.
http://www.gutenberg.org33,000 free books on their site, links to many more on other sites.
No PDFs; some are plain text, many HTML. There are free HTML-to-PDF tools available which work well, They also have versions of many for Kindle, iPad, etc. Eric downloaded obe of those and converted to PDF; maybe he can fill in details or point to the thread where he mentioned it.
Nothing recent, only older stuff whose copyright has expired. Complete Sherlock Holmes, Jane Austen, Dickens, ... This means it is all legal to download, give students copies, ... and you cannot get in trouble for it if Customs search your laptop on the way home.