Welcome to WebmasterWorld Guest from 22.214.171.124
There is one incident that leads me to believe those links pass PR, as it was the only link I could find that could have done it. But I could have missed another one, so I would not declare it to be true with any certainty.
I do not recall ever seeing a PDF listed as a backlink. It doesn't mean that they aren't, it just means that I haven't seen it.
It does seem to me that it is unlikely that they would put significant effort into using different valuations for links from PDFs when the easiest thing to do would be to just plug them in to the same formulas as the links from HTML documents.