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:
bToggleAdobeDocumentServices
bToggleAdobeSign
bTogglePrefsSync
Here is the article;
https://www.adobe.com/devnet-docs/acrobatetk/tools/VirtualizationGuide/singleinstaller.html#