If you have Adobe Pro on RDS installed but not all users have licenses, you need to have reader activated.
You can try install reader also. But the solution is simple.
Add the regkey below
HKEY_LOCAL_MACHINE\SOFTWARE\Policies\Adobe\Adobe Acrobat\DC\FeatureLockDown:“bIsSCReducedModeEnforcedEx”=dword:00000001
Now if a user has a license, Adobe Pro is active,
if a user dont have a license, reader is active.
Source: pdf – Installing Adobe Pro and Adobe Reader on the same machine – Super User