The short answer is that two or more files are needed to use Proximity, but I haven't figured out how to use it yet. This thread explains a lot: Acrobat search. Two words close to each other.
Here are the search abilities: Adobe Acrobat X Standard * Advanced Search Options
Also, in Preferences one can set the Advanced Search to always show more options.
HOWEVER, with the help of George Johnson, in Acrobat Professional one can build an index of the entire manual (just put it in its own directory). Search of that index found 3906 instances, so I guess I'm going to have to limit my search criteria!