1. An image forming system including a data processing apparatus that creates document data for an image to be formed and displays a preview image corresponding to the document data on a display, and an image forming apparatus that forms an image based on data acquired, wherein the data processing apparatus comprises: an image data generator that generates image data for the preview image; a setting receiver that sets post-image-formation processing conditions for the document data; an image modifier that modifies the image data for the preview image, according to the post-image-formation processing conditions received by the setting receiver; and an image display that displays the preview image on the display, according to the image data generated by the image data generator or the image data modified by the image modifier, and the image forming apparatus comprises: an acquirer that acquires the data acquired for image formation from the data processing apparatus; and an image former that forms an image based on the acquired data for image formation.