Doesn't work at all on Firefox 11, Mac OS Lion Rated 1 out of 5 stars
As a quick test, I went to http://www.irs.gov and clicked on the instructions for form 1040. This file renders fine using native tools (Preview, etc.) With pdf.js, the fonts are magnified to insane sizes, causing everything to overlap into a giant black mess on every page.
Maybe a future version will work, but this one doesn't.