I think I disabled it a while ago due to the local file disclosure issue, but I can't be certain. Does the latest stable TBB (5.0.3) have this enabled by default? It's really handy to read PDFs with pdf.js, but I don't want it enabled unless it is supposed to be.
Yes pdf.js is enabled by default. It is preferred to use this rather than downloading the actual pdf. In general pdfs tend to be a security risk though.
Although old, here is some more information regarding pdf.js in TBB pdf files opening on new tab on-click, is it safe?
Note: I would suggest using tails if you are viewing PDFs over Tor often.