Disable Adobe Sign-in - Acrobat DC
I ultimately fixed the Adobe Sign In pop-up issue by creating a license file package with Adobe Creative Cloud Packager and applying that after installing the Acrobat DC package.
This doesn't seem necessary with the other Creative Cloud products from what I can see.
Create the license file package and save it to a location.
Then enter the key and build the package.
After installation of Acrobat DC, you can run the AdobeSerialization.exe program located inside the License File folder created earlier to properly license Acrobat Pro DC.