1. An information processing apparatus, comprising: means for analyzing a layout of a document image, the layout comprising a character line having a plurality of intercharacter spaces each having an original length; means for embedding additional information into the character line by changing the original length to an optimal length for each of the plurality of intercharacter spaces, wherein the optimal length is obtained by performing an information embedding operation comprising the steps of: converting the original length to a quantized length for each of the plurality of intercharacter spaces using the additional information; obtaining an accumulated value representing a sum of the differences between the original length and the quantized length for each of the plurality of intercharacter spaces; determining whether the accumulated value is within a range determined by a threshold value to generate a determination result; and adjusting the quantized length of at least one of the plurality of intercharacter spaces based on the determination result to make the accumulated value within the range, wherein the adjusted quantized length is the optimal length.