We wanted to send a thank you message to our donors using mail merge but also incldue a pdf of their itemized giving statements for the year. Currently, there is not option to add attachments which prevents us from using mail merge for this reason.
Another thing could be to add a field for the giving statement feature to mail merge so it's easy for organizations to send end of year tax receipts
A work around for this is to merge the letters in Word, go offline and send from your email, then go to your outbox where all of the emails have been merged, and add the attachment by hand to each email, and send. Takes time but it works. Also, keep in mind that emails with attachments tend to go to spam more often, so you may want to find a way to include it in the email itself.