A paper shredder includes an upper cover and a pressing structure. The pressing structure includes a base plate and a retractable member. The retractable member includes a push rod, a first sleeve, and an elastic element. The first sleeve includes a first hollow portion. The push rod is inserted into the first hollow portion of the first sleeve. The elastic element is disposed within the first hollow portion of the first sleeve. When the paper stack is contacted with the base plate to push the push rod, the push rod is transferred through the first hollow portion of the first sleeve and the elastic element is pushed by the push rod. Consequently, the elastic element generates an elastic force to press the paper stack.