Focusing only on the

case, two equal and opposite forces acting on it can be identified –

the downward weight of the case and the upward applied force by

the hand. However, these two forces cannot be a third law force pair

because they act on the same object.

On an overall inspection, you should realize that the reaction

force to the upward force of the hand on the briefcase is a downward

force on the hand. Then how about the reaction force to the weight

of the case? Since weight is the attractive gravitational force on the

case by the Earth, the corresponding force on the Earth by the case

makes up the third law force pair.