Attaching ONLY a PDF icon, not the open file to a mail message.

One page PDFs fill show in all their glory in emails. However, 2 or more pages will only show as a PDF icon as you'd like. So you can add a blank page with the following text "This page left blank" and you'll get what you want.

Or, put the PDF file in a folder and add the folder to the email.

Or, compress the PDF file and add the zip file to the email.