A paper feeding device is mounted in a shredder and has a casing, and a pushing assembly and a feeding assembly mounted in the casing. When a stack of paper is put into the paper feeding device, the pushing assembly pushes the paper forward and the feeding assembly automatically feeds some of the paper into the shredder to shred the paper into small pieces until shredding of the stack of paper is complete. Furthermore, according to a capability of a cutting mechanism of the shredder, the feeding assembly is adjustable to feed different quantities of paper at a time. Therefore, the paper feeding device for the shredder is convenient, time saving and efficient.