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.