It's a browser extension, but the benefit of web apps in general is portability. This one has the advantage (from the looks of it) of solving a very hard problem, that of being able to view and manipulate pdf files. AFAIK there is no other easy and free way to do this in Linux, for one.
edit: It doesn't appear to let you edit a pdf, just annotate. Which is still incredibly useful to be able to do alone or collaboratively.
The extension is currently only available for Chrome, but all the functionality is available at http://web.notabledocs.com/web/viewer.html - If you share your document, that's what the other people get. We actually work offline (including making comments) if you install the extension.
Cool stuff! Something I've been thinking about too. Open Source?
Not right now, but it's something we're considering - we certainly owe a lot to Mozilla's PDF.js, which is doing the PDF rendering for us. It's just tricky to decide to give away the code when you don't yet know what part of your startup is (hopefully) going to be valuable.