Do you use "Save As pdf" or do you actually use a virtual "Print to PDF printer?" If you "print to pdf" is this your own virtual printer, which one, or are you trying to print to the provided one?

Have you tried "Send Later" to see if the pdf attachment generates, or do you directly "send" and is that while Previewing (customer template perspective) or only the on-screen (programmatic) view of that transaction?

Have you tried the pdf fix?

https://quickbooks.intuit.com/learn-support/en-us/help-article/manage-users/troubleshoot-pdf-print-p...