A printing device for providing multiple documents of one type includes a data receiving portion for receiving an image data, a processor, and a printing module. The processor is used to process the image data to generate a first printing information and a second printing information. The printing module is used to print a first document and a second document according to the first and second printing information, respectively. The first printing information includes the image data and a first identifying information, and the second printing information includes the image data and a second identifying information different from the first identifying information. The first identifying information is composed of a first set of halftone pattern, a first set of lines or a combining information thereof. The second identifying information is composed of a second set of halftone pattern, a second set of lines or a combining information thereof.