Adobe PDF's missing from "Recent Items" in outlook
If you open PDF files from within the Adobe internal file explorer the files will not show up under “recent files” when you try to attach in Outlook. In outlook when you click “insert”, “Attach File” then look under “Recent Items” the list shows all recently opened items EXCEPT pdf’s. (All PDF’s are missing) This issue was a problem a couple of years ago but this seems to be a very recent change or issue.
Cause (as I’ve identified it)

If you open the PDF using windows explorer instead the files do appear in the recent file list. (see below)

After opening from Windows Explorer the file shows up in recent items list


