If you receive a message that includes a PDF attachment, Gmail has a new option that lets you view the file without installing a PDF viewer like Adobe Reader. Until now, Gmail converted PDF files to HTML and removed images, so they didn't look very well. The new option uses the PDF viewer from Google Docs, which lets you zoom and search inside the (first 100 pages of the) file.
Google should provide an interface for browsing your Gmail attachments, where you can find files and email them. That interface could be Google Docs, assuming that the plan to extend the filetypes is still on track.