Do you mean a native pdf viewer or any pdf viewer including the javascript one? It's very convinient to open pdf:s in the same tab and not having to download a file and open it in an external viewer.
But for small pdfs, the in-browser pdf viewer is typically faster. I use lots of PDFs and while I read most in adobe reader, I still appreciate the in-browser rendering: it's often good enough, and stuff like smooth scrolling and tabbed browsing actually works better, and the UI is less in-your-face. The windows PDF reader has (IMNSHO) ugly fonts, and no upside I can find whatsoever.
Turns out to display the document you have to download the pdf anyway. I had so many issues with this PDF in browser thing reported to me by users... And disabling it can be a hassle with firefox sometimes ignoring the setting or simply not offering the option. I don't know what purpose it is supposed to have but it only gave me more support requests.
