Thank you for your post, and I apologize for the late reply.
If a PDF file is created with FileMaker Go, it will reside in the same work area as FileMaker Go. When launching a PDF file, it calls the PDF viewer in iOS, and the PDF viewer has the option to open the PDF file in another application. This would need to be controlled by iOS; not FileMaker Go.
Does anybody know of a way to disable this option under iOS?