Thank you that helps :-)
But I'm still asking myself, why Adobe is so inconsistent between the applications Photoshop/Illustrator/... and Acrobat. Why not CTRL+"+"/"-"?
Or different: Can I change/adapt the short cuts individually?
What's odd is that the shortcut commands CTRL + "+" and CTRL + "-" work for me on my Windows machine and the same works using "Command" on the Mac.
The shortcut commands CTRL + "+" and CTRL + "-" are available in Adobe Reader and Adobe Acrobat.
Not at my system:
CTRL+"+" and CTRL+"-" (both on NumPad) have no visual effect.
CTRL+"+" with the "+" on the normal keyboard scrolls down like PageDown.
I use Acrobat 9 Pro 9.3.1 und Windows 7 64bit (but the same effect under 32bit).