When the cardboard is at rest, the net force acting on it must be zero. This means that the magnitudes and directions of the pair forces acting on it must be equal and opposite.

For example, if a person is pushing on the cardboard with a force of 100 N to the right, then there must be another force of 100 N pushing on the cardboard to the left in order to keep it at rest.

Similarly, if a person is pulling on the cardboard with a force of 50 N upwards, then there must be another force of 50 N pulling on the cardboard downwards in order to keep it at rest.