1. An image processing apparatus comprising: an acquiring unit that acquires image data representing a document image that includes characters; wherein at least some of the characters included in the document image represented by the image data acquired by the acquiring unit have undergone modification in a vicinity thereof based on an operator instruction; an extracting unit that extracts, from the acquired image data based on identification of the at least some of the characters in the document image that have undergone modification in the vicinity thereof based on the operator instruction, a character from among the at least some of the characters that has undergone a modification in a vicinity of the character for distinguishing the character from another character in the document image; a processing unit that processes the image data by reducing a density of the character other than the extracted character below a density of the other character indicated by the acquired image data to visually highlight the extracted character relative to the other character, wherein a density of the extracted character is maintained; and an output unit that outputs the image data processed by the processing unit to an image forming apparatus.