As /TestScreenName asked, is it a pdf password pop up or Adobe Reader DC is asking for Sign In password?
What you have explained is only possible when the end user is sending a password protected document to you. Because Adobe Reader doesn't ask for the password to view a pdf.
For testing, you may login on web Adobe Document Cloud and see if your Adobe password if working or not? Also, to understand the issue clearly you may share the screenshot of the password popup screen.