1. An apparatus for processing a document image, comprising: a generating unit to generate a luminance component image from the document image; an estimating unit to estimate a luminance image from the luminance component image; and an adjusting unit to adjust the luminance component image according to the luminance image estimated, wherein, the estimating unit estimates luminance values of pixels at least in horizontal edge areas of the luminance component image according to luminance values of pixels in a part of a background of the luminance component image, and if the estimated luminance values of the pixels in the horizontal edge areas are determined as acceptable according to a predetermined criterion, the estimating unit estimates the luminance image according to the estimated luminance values, and if the estimated luminance values are determined as unacceptable according to the predetermined criterion, the estimating unit estimates the luminance image by using a largest one of the luminance values of the pixels in each column of pixels in the luminance component image as the luminance values of all of the pixels in that column.