I finally figured out how to solve this problem after over a year of frustrating scrolling (page up and page down works, just not mouse wheel scrolling or the up/down arrow keys). The trick is that it only happens on the secondary display - and the solution is to switch resolutions of the secondary display to something different than it is and then back to the original.
I don't know how long this has worked, since I just did it, but it is working now. I will follow up to let you know how permanent of a fix this is. I got the idea for this fix from this discussion on the Word for Mac newsgroup.

