An automatic document feeder to be incorporated in a scanner document forwarding mechanism which comprises a reduction gear assembly driven by a motor and having at least one gear and a pair of mated rollers through which a document to be fed by the document feeder travels is disclosed. The document feeder includes a gear train having an input gear mechanically coupled to the gear of the reduction gear assembly of the scanner document forwarding mechanism via two idle gears to be driven thereby and an output gear mechanically coupled to the input gear via a pinion co-axially fixed to the input gear and engaging the output gear. A document driving roller is provided and co-axially fixed to the output gear to be driven thereby for engaging and driving the document to be fed into the scanner document forwarding mechanism. A document guide is provided close to the document driving roller for guiding the document therethrough toward the scanner document forwarding mechanism. A document sensor is provided to detect the existence of the document and generates a signal to turn on the motor upon detecting the document.