Image files in container fields (not PDFs) are simple as you can set up a layout to display the image file and then the same Save as PDF method can include them.
A single page PDF might be included the same way, depending on the FileMaker version and possibly the OS that is in use.
A multiple page PDF in a container field could not be included in such PDF unless there is a plug in available to add that capability.
Yes its the multi page PDF documents that create the problem. Unless I can solve that part of the equation, I cannot get the whole process to work.
I don't think that you'll be able to do this just from FileMaker scripting.
You never know what a plug in developer might come up with and it might be possible to perform system level stuff such as windows command lines to manipulate a different application such as an ADOBE application in order to merge the PDF's into a single document.