A printer includes an enclosure, a paper output tray, and two positioning devices. The enclosure includes a receiving base and two side portions extending from the receiving base. The paper output tray is pivotally mounted between the two side portions. The two positioning devices are mounted to the two side portions. The paper output tray is placed in the receiving base in a first position. The paper output tray can rotate to resist each of the two positioning devices and move to a side portion, thereby enabling the paper output tray to rotate to a second position. Each of the two positioning devices restores to an original position to support the paper output tray stably in the second position, to enable the removal of a paper jam or other circumstance.