I've run into three closely related problems with
Communicator.
First, application types listed as "Unknown:Prompt User" in
the helper applications list don't seem to do this.  (It
saves to disk instead.)
Second, when I specifically change the application, it
doesn't seem to "take" (is forgotten the next time I return
to the list).
Finally, because of these I can't seem to find any way to
get Communicator to show PDF files, rather than saving to
disk.  I've verified (using wget -S) that the file type is
being reported as application/pdf by the servers.

I'm probably just doing something dumb, but I can no longer
figure out how to simply "ask a question" at this site, so
I've resorted to using Bugzilla... hope that's okay.