Sorry for the delayed response!
Well it seems that this is an issue with Acrobate 9.x in our system. I installed 10.x
and now I can comment the schematic. Unfortunately the marking ability of 10.x is
restricted to bubbles and marking text. In version 9.x you could draw also rectangles
in order to mark i.e. scanned text.
If you are free to choose another PDF reader you might use Foxit Reader. This program,
at least the latest version I tested, still has this function