PDF Files are mostly the e-books and others like the company notes that every web or the topic related learner needs. There are thousands of e-books, tutorials and guides available in the form of PDFs and searching them in normal search engines like Google, Yahoo, Live etc. would need some tricks like specific search for the .pdf extensions etc.

There are some specialized search engines only for PDF files, which list all kinds of e-books etc. related to every category.
Here are the 2 best i could find to share -

Pdf Search Engine - PDF Search Engine is a book search engine search on sites, forums, message boards for pdf files. You can find and download a tons of e-books but please respect the publisher and the author for their creations if their books copyrighted. Searching in this site is too simple and everything is perfectly set. There is also an option to add the search engine to the firefox toolbar where the other search engines are already present in the search box. Check the PDF Search Engine at pdf-search-engine.com

PDF Geni - PDFGeni is a dedicated pdf search engine for PDF ebooks, sheets, forms and documents. Any category PDF can be searched for, and also there is a section for hot searches, which can be helpful to see what topics are searched more and which are hot in the website.
Check the PDF Geni Search Engine at pdfgeni.com

There are hundreds of other PDF and ebook search engines but these two have produced the best and relevant results for my needs.