A duplex automatic document feeder includes first, second, third, and fourth paths, an ejecting path, a scanning module, first and second reversing roller units, a feeding tray, and an ejecting tray. A document is moved from the feeding tray to the scanning module for scanning of a first side surface thereof. Subsequently, the document is moved along a second-side scanning path defined by the first reversing roller unit, the third path, a portion of the first path, the fourth path, and the first reversing roller unit to allow for scanning of a second side surface thereof. After the first and second side surfaces are scanned, the document is moved along a document-inverting path defined by the second reversing roller unit, a portion of the fourth path, the ejecting path, a portion of the second path, the first reversing roller unit, and the ejecting tray.