where I can download free chess pdf books

Most sites hosting pdf's of books, are not legally doing so, at least not based on US laws. This being a US based site, people can't really share them here (or if they do, it will get moderated).
There are some public domain chess books out there, some of which are no longer under copyright. A google search will find those quickly.
"... In this column I want to give an overview of the sophisticated e+Chess app by e+Books ... The app itself is free from the Apple itunes store; what you pay for are the books you choose to download. ... You get a free sample book, Capablanca's Chess Fundamentals, and an interactive User's Manual. ..." - IM John Watson (2013)
http://theweekinchess.com/john-watson-reviews/john-watson-book-review-107-of-eplus-and-chess-praxis