I notice there is already a discussion about this. However, the website referenced with a solution was no longer available, so thought I would post my own solution.
Combining the PDFs that are stored in container fields would seem to be a fairly useful feature, given how often we all use PDFs these days. However, not possible with Filemaker. After hunting around on various forums I found a way to do this with functionality native to MAC, via Applescript. It has been done before, but it's a bit ugly for a novice and I wasn't able to find something that was bundled into a script.
The attached file demonstrates this functionality and provides scripts you can copy to your own solution for this purpose. It does NOT work on Windows installs of Filemaker, mainly because Windows does not have the Python script it uses. But this works seamlessly on MACs. So if that's all you want to do, it amy be useful. I have a one client who uses it to create quite large documents and he has been using it for a year without issues.
I hope it saves some of you the headaches!!