1. An image forming system including a data processing apparatus for creating document data for an image to be formed and displaying a preview image corresponding to the document data on a display section, and an image forming apparatus for forming an image based on data acquired, wherein the data processing apparatus comprises: an instruction receiving section for receiving an image formation instruction for the document data; a setting receiving section for receiving setting of post-image-formation processing conditions for the document data; an image data generating section for generating image data for the preview image, according to the post-image-formation processing conditions received by the setting receiving section, when the instruction receiving section receives the image formation instruction; a modifying section for modifying the image data for the preview image, according to the post-image-formation processing conditions received by the setting receiving section; an image display section for displaying the preview image on the display section, according to the image data generated by the image data generating section or the image data modified by the modifying section; and a formation data generating section for generating, based on the image data for the preview image, data for image formation which is to be sent to the image forming apparatus, and the image forming apparatus comprises: an acquiring section for acquiring the data for image formation from the data processing apparatus; and an image forming section for forming an image based on the acquired data for image formation.