Google rasterizes the PDF and streams it to you as an image. Scribd will be converting documents to HTML and CSS while maintaining a near perfect facsimile of the original document.
Plus, it will maintain the fidelity of the document -- meaning that even PDFs with complicated layouts will be rendered properly in HTML. No trivial task.
It's a great technical challenge, but will users notice the difference?
Once this is live, Scribd will gain at least myself as a user, and I suspect many more.
(Nesting is too deep to reply to axod, so: the fact that Google is doing this should be reason enough. Scribd exists as a place to publish material. That material should be reachable by as many users as they can manage.)
Works fine for me :/
Especially if your users are disabled.
I don't know how Scribd is going to carry this off, what with people sometimes uploading outright scans of books. I mean, Scribd is not Scribd without the stuff put there by people -- like with Youtube.