It also works as its own page: http://docs.google.com/gview?url=
Does anyone know if firefox extensions can register as handlers for MIME types, or if they're restricted to diddling with chrome / pre-existing pages? It'd be really easy to write a WebKit plugin to do it, but not very useful since Safari ships with a flawless PDFKit-based viewer.
I want to get this working in Firefox, but NPAPI plugins are really gross. It looks doable that way, as it appears that a naked nsplugin still gets a DOM and everything, so I could just immediately set window.location.href and be done with it.