How big are the PDFs? Can you try placing in a relatively small (~10-25kb) PDF file to see if that loads?
From my experience, sometimes the container will stay grey while the PDF loads, then display once loaded. On large PDFs this can take a while.
I've been down this road with FileMaker already.... this is a known issue.
thier suggestion was to make the server available to the public (I don't have specific technical recommendations at hand), but the client's IT department went wacko when they heard FileMaker's suggestion.
Last time I spoke to FileMaker in Australia, it was logged as an issue and to date, I have not received a reply. Hopefully, someone from FileMaker may be able to shed some light on the progress on this one.
The PDF progressive download uses port 16000. If your IT department is not willing to open this port (you can use port forwarding to the FMS computer to limit exposure) then there isn't anything you can do about it. That is the reserved port for FMP. No one else should be using it and as far as I know there are no known exploits on it.
The FMS Port listing can be found here:
You didn't say which version of FMS you have. Also, there is a known bug in the PDF viewer plugin where the FMP container goes blank (I think FMP 13v4 has a fix in it but I don't know how far back it works). You should be using PDF viewer 11 or greater.
I am using FMS13.
I will check with my IT about the status of port 16000.