Couple of possibilities:
1) instead of just opening the page that shows that entity use a microflow where you retrieve that filedocument first and then pass both objects as parameters to that page. You are then able to show both objects.
2) On the page use a datasource microflow that retrieves the filedocument.
3) Set a reference between the objects so that you can based on that object retrieved the associated file document.
Regards,
Ronald