When I click a PDF or Word attachment in Outlook the preview pane says "This file cannot be previewed because there is no previewer installed for it". It worked until last week. Windows 11, Office 365.

Attachment previews depend on the program that opens that file type, so check these:

1. File > Options > Trust Center > Trust Center Settings > Attachment Handling. Make sure "Turn off Attachment Preview" is not ticked and that the file type is not disabled under Attachment and Document Previewers.
2. For PDF, open Adobe Acrobat or Reader > Edit > Preferences > General and tick "Enable PDF thumbnail previews in Windows Explorer". Update Acrobat if it is old.
3. Set the right default app. Settings > Apps > Default apps, and confirm .pdf, .docx and .xlsx point to the programs you want.
4. Restart Outlook after each change.

If you installed PowerToys recently, its preview handlers can take over from Outlook's. Turn off the file preview module there and test again.

PowerToys it was. I had installed it two weeks ago. Turned the preview module off and the previews work again.

Good catch. That one confuses a lot of people because nothing in Outlook looks wrong.