Google has introduced the ability to search inside attachments contained within Gmail. Adding the prefix "has:attachment" followed by your keywords into Gmail’s search box will prompt Google’s algorithm to scour documents and presentations as well as PDF files. Queries can also be limited to specific file types by using the "has:attachment filename:extension" format. Currently, searches seem to be restricted to files that have been indexed by Google’s servers, so older documents may not be searchable just yet. Recently received attachments, however, should be good to go.