A method includes the steps of rotating an input roller to transport a first leading edge of a first document from an input tray to an input passageway, a scanning position and an output passageway sequentially; rotating the input roller to transport a second leading edge of a second document from the input tray to the input passageway after a first trailing edge of the first document deviates from the input roller; rotating an output roller to transport the first leading edge from the output passageway to an output tray; and generating a linear speed difference between the input roller and the output roller, after the first trailing edge deviates from the scanning position, to transport the first trailing edge to the output tray, such that the second leading edge is still within the input passageway or the input tray when the first trailing edge deviates from the output roller.