Sometimes it happens that you send a PDF through gmail and want to look it back. Till now, if you want to view the pdf attachment, you have to download the attachment. This is something irritating. People don't always need to download and save it right then.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. 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.