Catalina Mail Attachments Embed Issues

Nothing is going on. If the PDF is a single page document it is displayed in the sender's message but is still going out as an attachment. Multi-page PDFs, however, are shown as an attachment.

I recommend always sending documents from a Mac to Windows as a ZIP archive to avoid unwanted translation problems. A safety precaution, you might say.

Outlook doesn't know how to process real email properly. It never has and I doubt it ever will. Microsoft only considers Exchange to be real email, so everything else is just something they don't want to bother with.

The best way to overcome the problems with Outlook is to send the message as plain text (Format menu), and set these two options in the Edit > Attachments menu:

Send Windows friendly Attachments

Insert Attachments at End

You can set those as defaults by setting them from the Message Viewer window and not have a new message open. They then appear with "Always…" prepended on the command names.

Until Microsoft decides it should follow email standards, people who use real email clients will have to make adjustments when sending messages to recipients who use Outlook.