For some purpose maybe you want to search any PDF file that contains some information for free. I think best way to find it is through search engine. Google is the best one search engine with modification search, you can easily find some PDF file.
For example you want to search information, tutorial about jquery so in Google just type “filetype:pdf jquery” without double quotes. Then Google will provide you some PDF file that you can read or download for free.
Google is general search engine that you can use for special purpose, but have you ever know that there is a special search engine for finding PDF file? I recommend you PDF search engine in www.ebooks-search-engine.com that looks clean and so simple.
Different result with Google but I feel it this engine provides good and useful resource even it provides direct link to download. I hope this information will make you comfortable with your internet life.