>> To be sure, Safari is invoking a system level PDF reader to handle inline PDFs. That's what gets you that silky performance and nice rendering.

Which, IMO, is the way it should be, and not just for PDFs. I'm impressed by the fact that Mozilla implemented a capable PDF reader in JavaScript, but I won't be using it. Firefox and Chrome are both trying too hard to duplicate everything my OS already provides: their own video codecs, audio codecs, image codecs, the Flash plugin (Chrome), now a PDF viewer, and so on. I'd rather just have my browser render HTML/CSS, run JavaScript, and that's about it. It doesn't have to implement anything else itself, all modern OS's should have everything required to display all the embedded content.

The main problem I see with that is security. PDF is a huge attack surface, reducing that to the already existing and well tested javascript surface is a huge win. Ideally (in my mind) every browser will come to use Mozilla's pdf implementation.

Ideally, I would trust my OS vendor and be safe using their superior PDF renderer. I think my ideal world is likelier to come to pass than yours, sad to say.

That only works if you don't care about fragmenting what features you support on multiple OSes, and trust those codes to do the right thing in all of your use cases for it.

For some of the browsers it's clearly better for them to bundle the kitchen sink, since at least it's their kitchen sink and guarantees the kitchen sink will always be there.