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We use PDFX to email invoices. This month we have an insert going into all of the mail (in the form of another PDF file). Unlike the invoice, the insert is the same for every email.

Is there a way to attach another (pre-existing) PDF file when I create the invoice pdf or do I need to recode their PDF file into a GDI script?
Yes, assuming you are on build 1383 or higher, and are using method (aka Type) 4, you can add up to 15 additional attachments (any kind of file, not just PDF) to the email via //PDFX,Email.AttachFile, directives.