I don't have a compatibility chart for the versions of Acrobat and RoboHelp, not sure if that could be an issue. I assume Acrobat is working otherwise - for example, if you PDF from Word
It was the case that RH had to be installed before Word. I don't think that extended to Acrobat but it could be worth trying an uninstall and reinstall.
Also try setting Acrobat's compatibility settings to something low such as Acrobat 5.
Ultimately you may have to settle for generating to Word and creating the PDF from there.
See www.grainge.org for RoboHelp and Authoring tips