Acrobat Reader may need a desktop to even run, even if it does not create a visible UI. It's likely that it's starting and just freezing. There was an earlier thread on this where one of the suggested solutions was to allow the service to interact with the desktop, but this probably won't work if there is no desktop session active at the moment. Eventually the solution was to use Foxit which unlike Acrobat Reader seems to work even when there is no desktop session. Here's the older thread: