Adobe did an update recently that combines reader and acrobat, so if this is on a terminal server the reader users get prompted for login credentials.

Adobe published a way to get around that so users without a license can open adobe in reader mode, and if they have a license they can login.

Caring network ran into this on Friday because someone signed in with there credentials which caused all users to get prompted when they tried to open a pdf.

Here is the registry entry needed;

Additional required configuration

If Acrobat should launch in reduced mode AND you’d like users to access all Acrobat features via the menu Help > Sign in, then the following preferences must be set:

Option 1: Manual or scripted registry configuration:

• Set HKEY_LOCAL_MACHINE\SOFTWARE\Policies\Adobe\Adobe Acrobat\DC\FeatureLockDown\bIsSCReducedModeEnforcedEx=1
• Set HKEY_LOCAL_MACHINE\SOFTWARE\Policies\Adobe\Adobe Acrobat\DC\FeatureLockDown\cServices\bUpdater=1
• Delete any one of the following preferences or set anyone to 0 at HKEY_LOCAL_MACHINE\SOFTWARE\Policies\Adobe\Adobe Acrobat\DC\FeatureLockDown\cServices

bToggleAdobeDocumentServices

bToggleAdobeSign

bTogglePrefsSync

Here is the article;

https://www.adobe.com/devnet-docs/acrobatetk/tools/VirtualizationGuide/singleinstaller.html#