PDF Preview

Did you also enabled the "Preview Pane" in the Windows File Explorer?

Which version of Micrososft Windows are you using , and which version of Micrososft Outlook?

I am not sure if what you're asking is three different things.

Are you asking how to preview a PDF directly in MS Outlook in the Reading Pane?

If yes, accroding to Microsoft Support guidance, there's no built-in PDF previewer in Outlook.

You must download and install Adobe Reader DC and set it as the default program used to open PDF files (how to do this may vary between operating system versions). A restart of MS Outlook is required after this is done in order to preview PDF Files in Outlook.

OR,

If youbwere able to do this before and now you can't, are you asking how the "preview attachments" feature in MS Outlook would allow to open PDF attachments in a separate program when you click the attachment icon in the reading pane?

OR,

Are you asking how to preview a PDF fle when you're browsing for documents in your operating system, that were previously downloaded as an attachment from MS Outlook onto your computer disk?

In any case, Microsoft Support guidance also suggests that  if the attachment is within the message body, such file can't be previewed; you'll need to manually open the attachment.

Please clarify what exactly is the workflow that you're trying to achieve, and which versions of the programs are you using.