1. A document processor comprising: image information input means for inputting an electronic document described by a page description language and having a black-and-white image to be processed, the electronic document containing at least one control word that describes a black-and-white mark of the image; mark detecting means for detecting the at least one control word from the input electronic document; image portion designating means for designating a portion of the image based on the at least one detected control word; changing method determining means for determining a manner in which a visual characteristic of the designated portion of the image is to be altered; and image information generating means for generating a changed version of the electronic document by inserting at least one additional control word into the electronic document, wherein the visual characteristic of the designated portion of the image in the changed version of the electronic document is altered in accordance with the changing method determined by said changing method determining means.