"There was an error opening this document.
This file is already open or in use by another application."
Seems to be some kind of race - sometimes it opens but usually I get the above error. If I then click on the file in the browser downloads it opens just fine.

I just spent a couple hours talking to a very helpful support person in India and she was not able to fix the problem. Seems I will have to use a different browser or a different .pdf file viewer.
I am using the latest version of Firefox and the latest version of Adobe Acrobat Standard 2017 on a new Dell computer running Windows 10.