1. An image forming apparatus, comprising: an image scanning device configured to scan a document image of a document; an image type identifying unit configured to identify a type of the document image scanned by the image scanning device; an operation detecting unit configured to detect an image quality adjustment operation by a user; a preview processing unit configured to select one of whole preview displaying and partial preview displaying on the basis of the type of the document image and a type of the image quality adjustment operation, and perform the selected whole preview displaying or partial preview displaying, the whole preview displaying indicating a whole of the document image, and the partial preview displaying indicating a part of the document image at a higher resolution than a resolution of the whole preview displaying; and an image processing unit configured to perform image quality adjustment of the document image according to the detected image quality adjustment operation.