Same issue with Adobe reader DC.... here's what I did:
Uninstall Adobe Reader DC
Go to the adobe download page and download the previous version Adobe Reader XI
Make sure it's set as the default application for opening PDFs
Then, what was happening was that I would click on an imported item to sign off, and it would open it in a separate window.... So I went into my internet explorer browser and went: Tools --> Internet Options --> Programs --> Manage Add-ons --> Clicked on Adobe Reader and Disabled it, then went through the whole sequence and re-Enabled it, and now everything is working the way it was. NOt sure if this will work for everyone, but worth a shot... I'm using windows 10 and AC 9.x. Good luck
