Users will be able to easily view PDFs on the web from any device. I primarily read HN on my phone. Any time I see a [Scribd] link, I forget about the story because I can't read it. I have also had several occasions where I needed to read a PDF on the go. I had to email it to a friend and then call them, dictating the pertinent bits over the phone.

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.)

Although the semantic web is still pie-in-sky-land, having your content structured instead of a big block o' text is always better.

Especially if your users are disabled.

This is a little indirect, but presumably the googlebot will find it easier to index scribd pages, which might help you as a user if you're searching for something that's hosted on scribd.