Comment 25 for bug 344814

If there's no bold weight, we "fake" it by drawing twice with a slight offset. You should be able to tell the difference between this and a true bold face by zooming in; the fake bold will appear relatively less noticeable as the size increases. (Aside: we really should be using a better technique for this.)

I'm suspecting that it may be this fake bold effect that's not working in the PDF output.