1. A digital image forming apparatus, comprising: an image reader for reading images of a plurality of pages of documents and outputting an image data of each image; a printer for forming images on both sides of a paper sheet based on the image data inputted by the image reader; array setting means for setting allotment of each of paper sheets for the documents read by the image reader so that prints for saddle stitching are obtained by the printer and that an image of a specified document among documents is arranged on a spread page of the saddle stitch type prints; and control means for changing an order of the image data outputted by the image reader and inputting the image data to the printer in the changed order based on the array setting set by the array setting means.