PDF is the Portable Document Format used by Adobe Acrobat. It is designed for brochures, magazines, forms, reports and other materials with complex visual designs
which will be printed on PostScript (tm) printers. PDF is the best format to publish our books online.
For Different content we use different kind of search engine's services. Like in Google we use Google search for content, Image Search for Images and Video search for Videos. And here is Pdfgeni.com a perfect search engine to find only Pdf's online. To find any PDF of your interest Just Enter the keyword related to your search and it will show the link like Google Search Engine to Related content. For example I tried to search a PDF for Make Money Online and it showed the result as given in the below picture.
Looks good! try it...