1. A document authentication method implemented in a data processing system, comprising: (a) obtaining an original grayscale image representing an original document; (b) separating halftone or light text areas from non-halftone or dark text areas in the original grayscale image; (c) separately binarizing the halftone or light text areas and the non-halftone or dark text areas generated by step (b); (d) down-sampling the binarized non-halftone or dark text areas generated by step (c), whereby a binarized original image is generated; (e) obtaining a target grayscale image representing a hardcopy target document; (f) separating halftone text areas and non-halftone text areas in the target grayscale image; (g) separately binarizing the halftone text areas and the non-halftone text areas generated by step (f); (h) down-sampling the binarized non-halftone text areas generated by step (g), whereby a binarized target image is generated; and (i) comparing the binarized target image with the binarized original image to determine whether the target document is an authentic copy of the original document.