1. A method for embedding a digital watermark in a text document, comprising: (1) preparing a bitmap image constituted by dots, wherein the dots comprise coding dots; (2) obtaining the watermark and pre-processing the obtained watermark; (3) embedding the pre-processed watermark in the bitmap image, wherein the pre-processed watermark is represented by the shifts in positions of the coding dots; and (4) printing the text document including the bitmap image embedded with the watermark.