When you double-click on a document, that results in some app launching (whatever app is the default for that document type), and that app, at least while launching and opening the document, is in focus.
If you want to keep the finder in focus, then don't launch another app. You can view a document without launching another app by simply selecting the document, and pressing the spacebar. That's using Quickview, which is native to the system, integral to the Finder. When you view a document through Quickview, you will then have the option of opening it from that window, by launching its default app from that quickview window.
I think Gregg2 was probably curious (as I am) to know why you want to open a document, yet not change the focus from the Finder. That's always just a click away, in any case!
You were asking in your first post that you need to be pointed in the right direction. I think Gregg2 was offering the idea that a compass does exactly that, but that compass function does help you with what you are asking.
But, the question remains - Can you tell us what advantage there would be for you when launching an app, but keeping focus on the Finder?