By double clicking a .pdf Acrobat is launched. I've had two instances where Acrobat did not launch.
Probably there was something wrong with the pdf. Tried again and again. Nothing, Acrobat wasn't launched.
In the Task Manager > Processes, expanding Opus, the Acrobat instances were listed.
So I did a File > Exit, Opus was gone as well as the Acrobat entries.
Gave it a try, using some other file, but Acrobat still did not launch.
It appeared that Acrobat was still listed a few times at the Task Manager > Details tab.
Ended these tasks. Now Acrobat launched properly (whilst using the 'other file').
I am sorry for not knowing whether this is a Windows thing, or whether it is up to Opus to also end the tasks under Details? Opus started the launch of Acrobat. Under Processes the Acrobat entries are gone after a File > Exit.
Again, I don't know.
Thanks.