Hi Krishna,
If the only entry in the "Order Number" folders would be the "Final Order" PDF, the following advanced search syntax may work:
{LF:ChildName<>“Final Order”, Type=“D”}
<> is the "does not equal" operator.
If there are any documents in an "Order Number" folder in addition to the "Final Order" one, the folder may still be returned (because the search query would evaluate to True for the other document). You'd have to test that out to check the behavior. If you test it, please reply back with your findings to help others in the future.
The LF:ChildName (and LF:ParentName) searches can only be performed using advanced search syntax, so you won't find them in the search filter UI.
I've attached the Advanced Search Syntax white paper where I found the LF:ChildName search type. Though the title says "10.3", it's still valid for the latest versions of Laserfiche. Advanced search syntax hasn't changed.
Hope that helps,
Sam