Here's what Gmail blog reported :-
When I get sent a PDF, sometimes I just want to view it -- I don't always need to download and save it right then. So starting today, you'll see a new "View" link next to PDF attachments you get in Gmail:
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. Here's a screen shot:
If you want, you can still view in plain HTML from a link at the top of the new viewer. And if you want to download, save, and view your PDFs later while offline using client software, you can still do that by hitting the "Download" link.