Have you scripted the generation of the mail-merged letters, then the e-mailing of the attachment?
Should you not generate the mail-merge letter, send the e-mail with that attachment, and then loop that?
I am paranoid about scripting e-mails with attachments. The potential to send one e-mail with another's attachment gives me sleepless nights of 'check, check, and check again'.
And then check once more. To be sure.
To expand on Sorbsbuster's answer, you can save as PDF with the current record option to get just the PDF for the current record. Inside a loop, that will enable you to send out one email at a time with the correct, attached PDF.
And when I write a looping script like that I usually start it with generating an inocuous pdf to over-write any existing pdf, in case at some early stage the user cancels the script, and the next time a pdf from the past looms up like a ghost. It's not Filemaker, by the way: it's me. Laugh if you want.
Okay: you can stop now.