I've found pdf.js to be a negative user experience on most pdfs I try to view. From what I've read, this is mainly because pdf.js directly renders to a canvas, and doesn't store decoded vector information. Other readers seem to be able to zoom instantly, even for vector graphics.

Firefox also seems to register two separate mime-types for pdf, only giving an option to use pdf.js on one of them. I've yet to dig into firefox and fix this.