1. An image forming apparatus for embedding digital watermark information into an image data of a document and outputting the image data, comprising: a first determination unit configured to determine a selected page of the document to embed the digital watermark information by comparing characteristics of each page with a predetermined threshold when the document is composed of two or more pages, the characteristics including a number of characters on the respective page; a digital watermark embedding unit configured to embed the digital watermark information into the selected page of the document determined by the first determination unit, an image outputting unit configured to output the image data which includes therein the digital watermark information embedded by the digital watermark embedding unit.