A duplex document feeder includes a housing unit, and a switching unit. The housing unit has a document-conveying path, an inverting path, and a switchback path. The switching unit is disposed in the switchback path, and includes a first switching gate, a second switching gate connected fixedly to the first switching gate, and a guiding path disposed between the first and second switching gates. The switching unit is pivotable among first, second, and third positions so as to allow for automatic feeding, two-sided scanning, inverting, and ejecting in orderly arrangement of documents.