A feeding unit from reels (B1, B2) of sheet-like material (F1, F2), in particular but not exclusively printed plastic film with print-position marks for automatic packaging machines, comprising two unrolling shafts (1, 2) of respective reels (B1, B2) of material (F1, F2), for feeding in turn said material towards means (7, 8) for cutting and welding the tail of material of a running out reel (B1) and the head of a fresh, loaded reel (B2). Such means comprise 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 (Z). Each device is formed by a group of mutually stacked suction and cutting modules, at least two of such modules having respective guides for cutting means 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.