1. A document authentication method implemented in a data processing system, comprising: (a) obtaining an original grayscale image representing an original document, the original grayscale image including one or more halftone or light text areas and one or more non-halftone or dark text areas; (b) separating the halftone or light text areas from the 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), which correspond to the non-halftone or dark text areas of the original grayscale image, without down-sampling the binarized halftone or light text areas which correspond to the halftone or light text areas of the same original grayscale image, whereby a binarized original image is generated; (e) obtaining a target grayscale image representing a hardcopy target document, the target grayscale image including one or more halftone text areas and one or more non-halftone text areas; (f) separating the halftone text areas and the 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), which correspond to the dark text areas of the target grayscale image, without down-sampling the binarized light text areas which correspond to the light text areas of the same target grayscale image, 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.