And here I found it much less bloated than the other free desktop PDF viewers.

I think Mozilla's line of thought here is that PDF documents are widespread in the web, to the point where they are a de facto web document type. So it makes sense for a web browser to support them rather than calling out to a user's desktop program (though I assume you can configure it to do so instead).

There's probably a bit of "our competitors do it, so we have to too" in there as well.