Good Evening, Richard.
"What am I doing Wrong?" - Nothing.
This often happens. If you want to attach something to an email and you don't want it embedded, easiest thing to do is put the attachment (in your case a PDF, could be a photo) into a Folder.
So, create a Folder, pop the attachment into that and add the Folder to the email.
Hope that helps. Please post back. We are here to help.
Ian