The PDF format is assigned to an unknown application

What causes this issue?

On your computer, the document format "PDF" is linked to an unknown
application. PDF integration is based on the appropriate Adobe products
(Acrobat Reader or Adobe Acrobat). The application installed on your
computer is not supported.
However, the R/3 application will now try to launch an appropriate
Adobe application or if all else fails to pass the document to the HTML
Viewer.

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?

Install the (free) Acrobat Reader, Version 4.0 or later.

Procedure for System Administrators

Error message extract from SAP system. Copyright SAP SE.