I've done this by roundabout routes.
There are chm-to-rtf converters around. I used one called ABC Amber CHM --
It produces (messy) rtf. I imported this into MS Word, cleaned things up, and then exported pdf. It's quite a bit of work, but anything's better than re-typing what Sandcastle produced.
Option 2: one of the other documentation generators (I think it was doxygen) can output either rtf or latex. Then, again, after some cleanup, I got what I wanted.
I just noticed that the Amber CHM thing can produce pdf directly. I haven't tried it, but I can't image how it could give good results.
If one were sufficiently clever and industrious (I'm neither), I suppose it would be possible to write XSLT transforms that output rtf or Office XML or Latex or something similar.