You have to install small module to help the Windows Indexing Service to extract information of PDF files. There is a pretty decent one: Foxit IFilter (Note: Foxit IFilter is either bundled to Foxit PhantomPDF to be used on a desktop or as a standalone server product). You can also use the one of Adobe (free).
After you have installed it, the Indexing Service is able to create a searchable index of all of your pdf-files (or at least the ones you have in folders for which you have configured WIS to index content :)). If the pdf get indexed, you are then able to search for content directly in the windows explorer.
Tutorial on how to install an IFilter: http://www.documentsnap.com/how-to-fix-pdf-search-in-windows-7-64-bit/