The way it works is pretty simple. If you’re the recipient of a PDF, you’ll now get a “View” link along with the previously existing “Download” link. When clicked, the “View” link opens up a new tab, which promptly loads the PDF in Google Docs, without opening up any desktop software (ie – Adobe Reader). Other features of the viewer include options to zoom in and out, as well as quick paging via next/previous arrows.
As the recipient of a lot of PDF files, I’m pretty stoked about this addition to Gmail. On the other hand, it’s a bit of a blow to startups Scribd and Docstoc perhaps, who offer their own solutions for web-based PDF viewing.
Also, and I’ll assume this is just temporary and give Google the benefit of the doubt, but the new PDF viewer doesn’t seem to work in Firefox, but works perfectly in Chrome.