You can't do what you can in Pro in Reader. I have never liked Reader since I have had Acrobat Pro. However, when an unknown or non-particular file was launched, it still was opened by Reader DC.

The association sort of worked when I wanted to open a particular file. When I re-associated it with PDF files, it was listed as an 'unknown application'. I had to search the hard drive for it because it was not listed as a program under 'All Apps''. Thank goodness Acrobat Pro 8 was still alive. Acrobat Pro 8.0 disappeared from the Default drop down menu and was longer associated with PDF files. Windows 10 replaced my default PDF program with Adobe Reader DC. Windows 10 recognized it and it had the proper file associations. Prior to a recent update of Windows 10, everything seemed ok with running Acrobat Pro 8.0. Acrobat Pro 8.0 was on the Win 7 Pro machine and carried over working.

My Win 10 Pro is an upgrade from Win 7 Pro. I don't know whether these steps have allowed my Acrobat Pro 8.0 to work in Windows 10.

Adobe reader 8 windows 10