I hate pay-walls and even login-walls, so I genuinely disagree with Scribd's payment model. However, I do think the service provides value in that users are able to upload a PDF and it renders in a pretty widely-compatible viewer format (in HTML5). Embedding PDFs across multiple platforms are still a terrible native experience, and on some systems it launches tons of painful Acrobat toolbars or just doesn't show up at all.

I have yet to find a free or open source solution that's incredibly easy to implement and embed (for bloggers). To get PDF content showing nicely inside of a scrollable iframe, you need to convert the PDF to HTML, and host images somewhere. That's not easy for people who just want to jump right into publishing blog content. DocumentCloud seems totally awesome, but their hosted platform is restricted to journalists; specifically, "newsrooms."

I use Scribd for legal research; there's a lot of attorneys who post PDFs of case pleadings, since (a) PACER is expensive to use, and (b) RECAP has terrible searching. To that end, it really is the YouTube of PDFs, and I love it for that. Of course, YouTube is ad-supported, so perhaps that'd be a better option for Scribd, but that tends to draw ire too.

For what it's worth, you don't need to pay for a Scribd account if you regularly upload content. I haven't uploaded anything in a few months and I was able to download, for free, the Declaration of Independence link that the author highlighted. Of course, you wouldn't know that unless you stared at the little text on the bottom of the page, so that should change.