An apparatus for treating a stack of sheets of sheet material includes a means for moving the stack of sheets of sheet material towards a first and second fold roller means. The first and second fold roller means are arranged to assist with folding of the stack of sheets of sheet material. The apparatus also includes a first and a second clamping jaw defining a gap. The first and second clamping jaws are arranged to be movable relative to each other to increase and decrease the gap for selective application of a clamping force to the stack of sheets of sheet material having passed between the first and second fold roller means. The first and second fold roller means are mounted in a manner allowing for an increase and decrease in separation and thus between a stack-guiding configuration and a stack compressing configuration.