This page requires a secure connection which includes server authentication

Many users in our organization receive a certificate popup when opening PDFs. They receive the alert, "This page requires a secure connection which includes server authentication. The certificate issuer for this site is untrusted or unknown". The same message will pop up a few times, whether the user hits yes or no. The issue remains even if they hit the view certificate and attempt to install. We've added the Adobe.io certificate to our firewall, but the issue remains. When looking into the matter, someone else mentioned they ran into a similar problem, but it appeared to be related to 2 different versions of Lightroom installed. When checking our users, none have any duplicate applications installed.