Gmail has some awesome advanced search features, and today it's become even more powerful: now, you can find old attachments by searching for text inside them.
Previously, you could only find an attachment if you searched for the message's text, the filename, or its file type. Now, however, you can search for text within DOC, PDF, PPT and other files and Gmail will bring them up in your search results. We tested it with a few PDF and DOC files, and it worked pretty well. It didn't find every file we searched for, but note that Gmail is still indexing all these attachments, so your mileage may vary as the feature rolls out. Hit the link to read more.
Search Inside Gmail Attachments [Google Operating System]