Can no longer view pdf files in viewer

I have used DO for many years and versions without a problem but now the viewer does not work with pdf files. Instead I get a red window in the view window saying, "This pdf document could not be displayed" The viewer continues to work well with other sorts of files such as png, docx or xlsx.

When this first occurred I was using Foxit however it still occurred when I set Adobe Reader as the system viewer. I think this relates to DO rather than the pdf readers because Windows Explorer does not have this issue. I'm using v12.28 Pro. Can anyone suggest a solution?

If the PDF viewer works in File Explorer but not Opus, go to Preferences / Viewer / Plugins, select ActiveX+Preview+Office+Web, click Configure, then Defaults (bottom left), then OK.

(Before clicking OK, it may also be an idea to note which PDF viewers it detects and which one the .pdf extension is assigned to.)

Does it work then?

If it still doesn’t work, but is OK in Explorer, other things to check are that Opus hasn’t been launched elevated (see UAC FAQ for more detail), and that antivirus isn't blocking things.

Reinstalling the PDF viewer may also fix its registry settings, if the issue is there.

Thank you for your suggestions
**Setting the defaults for ActiveX... did not work.
**The pdf extension was assigned to Foxit
**I was unable to get the viewer to work with the ideas you suggested but did succeed when installing Adobe Reader; the pdf extension is now assigned to Adobe.