The recipient receives two PDF attachments that are based on different templates. The first attachment is a letter derived from a Word document and the second attachment is a certificate derived from a Powerpoint presentation.
pdfMachine merge allows you to send multiple dynamically generated PDF attachments.
This is very similar to the simple mail merge example "A certificate of achievement is emailed to each person listed in the spreadsheet". The only difference is on the attachments page, you specify two dynamic attachments, one the powerpoint file, the other the word document.