Using older versions of Outlook for Mac for personal email and calendar
Went to Beta chanel, found 16.67. Send an email with S/MIME cert and PDF attachment. This time it was delivered. Strange behavior. I've seen posts of others who say that randomly attachments were sent with previous versions. Interesting observation with 16.67: I have a signature including a logo (PNG image) it wasn't delivered either before. But now the logo is in the signature and in addition to the PDF I sent attached as separate file.