Also left unexplained is why Adobe needs to constantly update Reader. The PDF spec has not changed in forever. Why should this software need updating if there are no changes in functionality or spec compliance?
Security problems as far as I know. Many governmental workers have it installed, so it's used as a spear phishing vector.
"Re: UN Resolution #XYZ (Attached PDF)" is how these things play out. You open the PDF and your computer develops, ah, magical new capabilities, like sending copies of everything you do to Eastern Europe or China.