Not much info here just someone with the same issue: “Yeah, this may be because the latest Reader installs itself as Acrobat DC and not Acrobat Reader DC. We lost PDF file association on our upgraded clients because .PDF is set to be opened by Reader (AcroExch) and now it expects opening by Acrobat DC.”
“Dear user, the fixes for this issue have been applied in the latest release so that Reader will not upgrade from 32 bit to 64 bit machines on Sever OS machines. You can get the latest version of 32-bit Reader from: https://get.adobe.com/reader/otherversions/
If you have installed the 32-bit Acrobat Reader DC application, you will be automatically upgraded to the 64-bit version of Acrobat Reader DC gradually. This upgrade is silent and currently applicable to a limited number of users. You are excluded from the automatic upgrade to 64-bit if you do any of the following:
Use an Enterprise or Federated ID to sign in to Acrobat Reader DC.
Customize Acrobat Reader DC using the Acrobat Customization Wizard and install it.
Install third-party plugins.
Disable the automatic updates in Acrobat Reader DC.