A more technical overview has the answers: http://blog.mozilla.com/cjones/2011/06/15/overview-of-pdf-js-guts/ http://blog.mozilla.com/cjones/2011/06/15/overview

A more technical overview has the answers: http://blog.mozilla.com/cjones/2011/06/15/overview-of-pdf-js-guts/ http://blog.mozilla.com/cjones/2011/06/15/overview-of-pdf-js... But tl;dr: "we currently plan on doing a fast first-paint of pages using canvas, concurrently building an SVG document for the page in the background, and when the SVG document is ready, switching to that."