I had to go through several people at RIM to discover a while back that scanned PDF's definitively will not show on a BB even with 4.0 BES & Attachment Service. Odd because they will display TIFF's, but there you have it.
There is a service -- pricey IMO -- which alleges to solve this problem. I have a user who is going to begin trying it next week. Check out http://www.terratial.com/