Answer:
The rubber band works as a spring, so the force that you need to stretch it would be equal to:
F = X*k where X is the change in the perimeter of the rubber band and k is a constant of the rubber band.
So you need to apply force to stretch it, and when you put it again in the stack of papers, the stack of papers applies a normal force against the rubber band that keeps the rubber band a little bit stretched.