You don't want to rely on the computing power at the client side, do you? :)

I guess one possible setup would be pdf.js running on server-side and having its output captured. One advantage of this, from what I can see, is that there would probably be fewer external dependencies than this setup.

Yes, actually they had this kind of plan, but I am not sure how it has been going.

It would be definitely interesting in that way, but in that case it may not be worth it to rewrite everything in JS.