Which 'mail merge toolkit'?
If you use
http://www.gmayor.com/ManyToOne.htm, presumably in one to one mode, you can merge to the body of an e-mail message and attach a common PDF to each, or attach a PDF identified in the data source, or you can merge to PDF and attach that PDF as an attachment with a standard message or a personalised message - or any combination of the above. And no messages are sent until you are happy with the resulting messages, which are initially created in a temporary Outlook folder.