A feeding unit from reels of sheet-like material , in particular but not exclusively printed plastic film with print-position marks for automatic packaging machines, comprising two unrolling shafts of respective reels of material for feeding in turn said material towards a unit that cuts and welds the tail of material of a running out reel and the head of a fresh, loaded reel. Such a unit comprising suction, cutting and welding devices (7, 8) movable so as to approach and move away from each other to press the material (F1, F2) over a welding plane. Each device is formed by a group of mutually stacked suction and cutting modules, at least two of such modules having respective guides for slidable in a transversal direction with the respect to the feeding direction of the material, whereby it is possible to select and adjust the number of modules according to a desired welding height.