I'm really excited for this for http://www.sharelatex.com and other similar sites that are actually generating a PDF for you. With native PDF viewers there is no way to interact with the viewer via javascript and even just having the viewer stay on the same page when your reload a document (with minor changes) is impossible. Pdf.js means that we'll be able to do this easily, as well as other cool things like letting letting the user sync between the PDF and source.
Are you guys actually generating PDFs from user input? If so, I'd be really interested in finding out how you do it if you don't mind sharing. I've found a number of apps that allow you to generate PDFs server side but often the installation is near impossible (dependency issues), e rendering is not good enough, or the API makes using it with a web app clunky. I did find a solution (don't recall the name) that actually used Webkit to render web pages and then turned them into PDFs but I was never able to get it to work in an app. It only seemed to work some of the time and only through ssh. But if you can't say, then I can respect that.
