As per the description above, I can understand that you are facing an issue in installing the Adobe Reader, Is that correct?
You cannot upgrade from Acrobat Pro Dc to a free version of Reader, you can install it and use it on your machine.
Assuming you are using a Windows machine. If you are unable to install the Reader back, then use the Adobe Cleaner tool to uninstall Acrobat Download Adobe Reader and Acrobat Cleaner Tool - Adobe Labs Link: http://labs.adobe.com/downloads/acrobatcleaner.html
Restart your computer and then install the Reader back.
Hope this helps, and keep us posted with the results.