I'm not sure if the fact that the scenario is running Outlook in a Windows Server 2016 Remote Desktop Environment with a User Profile Disk enabled for storing users profiles is relevant or not, but for transparency I'm starting with that information.
It also seems to occur if you log on to a host locally and not through the RDS environment.
Basically, the issue boils down to when Protected Mode is enabled, a PDF attachment in an email cannot be opened and fails with the error "There was an error opening this document. Access is denied."
I traced the problem down to being a write access issue on the Outlook Secure Temp Folder location C:\Users\<user>\AppData\Local\Microsoft\Windows\INetCache\Content.Outlook\<random string>\. It seems when opening the file in Outlook, the PDF is created there but as Adobe Reader tries to access it a write request is denied and then Reader fails to open it with the above error.
The version of Adobe Reader DC is 18.011.20040.19174
As per the issue description mentioned above, you are facing issue when trying to open pdf attachment from outlook, is that correct?
Could you try adding the folder path mentioned above under privileged location in Reader and check if that helps? Refer to the steps given in the following help document for the same: Enhanced security setting for PDFs, Adobe Acrobat
Let us know if you need any help.
Thanks for your suggestion. I will go ahead and give it a try and report back.
However, in an RDS setup where we have over 200 users, this would not be a particularly ideal solution as it would mean either a) entering over 200 unique privileged locations, or b) entering c:\Users\ as a privileged location which of course would weaken security.