1. An image forming apparatus that changes image data into document data comprising: a storage unit including the image data, the document data including character attribute data, and font matching data; and a control unit including at least one of a general purpose processor (GPP), a central processing unit (CPU), a micro processing unit (MPU), a digital signal processor (DSP), a graphics processing unit (GPU), or an application specific processor (ASP), configured for: recognizing, from the image data, an area in which one or more characters are drawn; calculating a vector of a line drawing of each of the one or more characters; judging a character code with respect to each vector by using an artificial neural network; setting, in the document data, the judged character code as the character attribute data; calculating a size of a bounding box serving as a drawing area of drawing data of a font of the font matching data corresponding to the character code; estimating a plurality of fonts or character sizes; performing matching by performing image comparison on either one or both of (i) one of the fonts and (ii) one of the character sizes; calculating a degree of similarity, wherein the image comparison is performed by setting windows of 8 to 192 dots in vertical and horizontal directions and calculating the degree of similarity of a luminance of a dark point of each of a pixel using dynamic programming; selecting the font and the character size where the degree of similarity is the highest; determining, if the font and the character size selected have the degree of similarity greater than or equal to a specified threshold value, as corresponding to the font and the character size and establishing matching; determining, if the degree of similarity is lower than the specified threshold value, that there are no corresponding font and no corresponding character size and matching is not established; setting each character of the character attribute data in the document data, to the font and the character size estimated; setting so that the character is located at a drawing position corresponding to the image data at the time of being viewed or printed, wherein an origin of drawing, a character width, and right, left, upper, and lower side bearings as the drawing position of the character so that the origin of drawing, the character width, and the right, left, upper, and lower side bearing correspond to the image data; calculating a subsequent origin where the character is subsequently drawn in accordance with the origin calculated in accordance with the character whose matching is established, wherein, if the subsequent origin and the origin of the subsequent character coincide with each other, the character whose matching is established and the subsequent character are set as the characters within an identical text in the document data; and setting, in the document data, a line drawing including no character or the drawing data of an image, if no matching is established with respect to the drawing position of the document data corresponding to the area of the image data, by determining as corresponding to none of the fonts and the character sizes included in the font matching data.