14. A document processor comprising: an input device that inputs an electronic document described by a page description language and having a black-and-white image to be processed, the electronic document containing character codes that describe characters in the image and mark codes that describe marks in the image; a mark detector that detects at least one specified type of mark code in the input electronic document; a character string designator that designates a character string in the electronic document based on the detected at least one specified type of mark code; a visual characteristic changing circuit that determines a manner in which a visual characteristic of the designated character string is to be altered; and an image generator that generates a changed version of the electronic document by inserting at least one control word into the electronic document that alters the visual characteristic of the designated character string in accordance with the determination made by the visual characteristic changing circuit.
14. A document processor comprising: an input device that inputs an electronic document described by a page description language and having a black-and-white image to be processed, the electronic document containing character codes that describe characters in the image and mark codes that describe marks in the image; a mark detector that detects at least one specified type of mark code in the input electronic document; a character string designator that designates a character string in the electronic document based on the detected at least one specified type of mark code; a visual characteristic changing circuit that determines a manner in which a visual characteristic of the designated character string is to be altered; and an image generator that generates a changed version of the electronic document by inserting at least one control word into the electronic document that alters the visual characteristic of the designated character string in accordance with the determination made by the visual characteristic changing circuit. 19. The document processor according to claim 14, wherein the visual characteristic of at least one of image portion forming lines or an image portion background area is altered.