A method and apparatus are provided, for forming a stack of interfolded sheets of porous material including a leading panel having a partial-panel width joined along a fold line to a trailing panel having a full-panel width, by controlling the position of leading edges of the sheets both upstream and downstream from the nip between the interfolding rolls of the folding machine with vacuum applied through corresponding vacuum ports disposed in the periphery of the folding rolls at circumferential distances equal to the partial panel length ahead of each gripper and tucker in the direction of rotation. Sufficient suction is applied to hold both the leading edge of the leading panel of a given sheet and an underlying portion of the trailing panel of the immediately preceding sheet against the periphery of one of the other of the rolls downstream from the nip.