When your friends sent a PDF files to you — you have two options now on your gmail inbox. Either you download the file or directly see the PDF file without leaving or using any third parties applications.
Marc says, “Clicking “View” quickly opens the PDF inside your browser, complete with the graphics and formatting you expect to see in a PDF. You may have seen this feature before, in Google Docs. It’s the way that we did uploading and viewing of PDFs online.”
Also, you can alternatively use any PDF reader and anytime you can download the files too and access offline mode.
Fast PDF viewing right in your browser [via gmailblog]