Gmail indexes the text in your attachments
Gmail Index (Update)
Google has integrated a new feature extremely useful search engine Gmail . The content of email attachments in Gmail is now indexed. This means that you can do a keyword search in PDF documents, Word and PowerPoint attachments is in your account.
Gmail has always provided advanced search operators, for example From: to find a specific sender, Filename: allows you to find a specific attachment type (PDF, DOC, JPG) or is: unread messages to display read. You also find a complete list of operators here . This innovation is a major breakthrough for Gmail users. It will allow them to better manage and especially to better disentangle the contents of their mailbox.
To search for keywords contained in an attachment, use the operator has: attachment + your keywords. Basically, the search engine will give you instant results from your attachments. It is possible that the feature is not yet enabled on your account. also find more details in Googl Mail Official Blog