Almost all search engines support searching inside PDF files and almost all of them have special keywords that the user can pass if you want them to return only PDF related results, but by default the results that you see for any query are mixed with regular web links as well PDF files.
If you’d like to see results from only PDF files from Google and don’t remember the special keywords for it, try Live PDF.
Live PDF is a free web service that lets you search PDF files from around the internet using either Google or Bing. The search engine to be chosen can be selected by the user.
The links that you get in the search results point directly to the PDF file so downloading the linked file is as easy as right-clicking and selecting the Save Link As option.