According to the Google Operating System blog, Google has added support into Gmail to search inside of attachments for relevant text. Previously, only attachment names or keywords from the email could be found, but now the contents of attachments, such as text in PDF documents, is searched as well.
The good news is that Gmail has finally added support for searching inside attachments. I've just tested this feature for .pdf files, .doc documents, .ppt presentations and it works, even though some old attachments may not be indexed yet.
To find messages that have attachments, search for has:attachment and add some keywords to your query. To restrict your search to PDF files, search for has:attachment filename:pdf.