Generating the HTML Viewer as an alternative failed

What causes this issue?

The R/3 System could not address the Adobe interfaces provided for R/3
integration. As an alternative, the system then tried to display the
PDF document in the HTML Viewer. This attempt also failed. The system
could not display the HTML Control.

System Response

The system issues an error message and will not allow you to continue with this transaction until the error is resolved.

How to fix this error?

Procedure for System Administrators

Error message extract from SAP system. Copyright SAP SE.