A printer includes a main body and a paper holding device. The main body is capable of printing and outputting paper. The paper holding device is secured to the main body, and includes a paper tray, a plurality of sliding members, a connecting mechanism, and a driving mechanism. The paper tray is configured for receiving the outputted paper. The sliding members are slidably located on the paper tray. The connecting mechanism is attached to the paper tray and connected to the sliding members. The driving mechanism is attached to the paper tray and capable of driving the connecting mechanism to slide the plurality of sliding members to shape a space to fit the size of outputted paper.