A document feeder includes a housing, a sheet pick-up arm and a stopper. The housing includes a sheet feeding path for placing paper sheets thereon. The sheet pick-up arm is movable between a first position and a second position. The sheet pick-up arm has a protrusion structure. The stopper is disposed under the sheet feeding path and includes a pivotal portion and a sheet stop portion. The sheet stop portion has a hooking part corresponding to the protrusion structure. When the sheet pick-up arm is moved to the first position, the protrusion structure is located in the swing track of the hooking part to restrain the swing range of the sheet stop portion. When the sheet pick-up arm is moved to the second position, the protrusion structure is deviated from the swing track of the hooking part to permit free swing of the sheet stop portion.