I have been reaming people out for years about sending me inaccessible PDF 
documents.
I got an important one last night, that I couldn't open with anything on my 
main computer, which is an XP machine.
I sent it upstairs to my windows 7 laptop, and it opened right up.
Well I completely removed and re-installed acrobat on the XP machine, and 
this time I got a dialog, after the accessibility settings dialog.
It said that in windows XP, some documents will not display if acrobat is 
set to be in protected mode.
Their instructions are not clear, or misleading at best.
You go to the edit menu, preferences, advanced security, and turn off 
protected mode.
The computer has to be restarted, and then the documents become visible in 
acrobat and windows xp.
H T Kaufman MSW LCSW
Adaptive Technology Instructor