I can't actually believe that they would go the paywall route. It honestly seemed a few months ago when they added HTML5 support that they were going in a really great direction and now I will avoid them like the plague.
I think the whole HTML5 thing was a big PR exercise to improve the way they are seen. Possibly also a big play to get acquired - "We're doing cool stuff with fonts! Buy us!".
Fundamentally though, they're about locking away documents behind a pay wall. And personally, I don't see their value proposition at all.
Their value proposition was to not lock documents away behind a pay wall. Even then they were not doing very well because of all the tag clouds and their irrelevant appearance on search results. They were the first though and do not have much competition. Quite an opportunity for someone looking for an idea.
I think it was just about "gaming Google" -- at some point the links to pages on scribd had higher ranks than original pdfs. I consider that more like successful spamming of Google search than long-term feature.
How is it now? Do they still have higher rank? I'd consider than that a bug in Google ranking.
Fundamentally though, they're about locking away documents behind a pay wall. And personally, I don't see their value proposition at all.