1. A method for authenticating a printed document, the printed document carrying barcode which encodes compressed image data representing a binary original document image, the method comprising: (a) obtaining an image representing the printed document; (b) separating the image into a target document image and the barcode; (c) decoding the barcode and decompressing the compressed image data therein to obtain the original document image; (d) binarizing the target document image; (e) aligning the target document image with respect to the original document image; (f) comparing each word in the original document image with a corresponding word in the target document image to detect any differences, comprising: (f1) for each word of the original document image obtained in step (c), finding the corresponding word of the target document image; (f2) generating a difference map and calculating a Hausdorff distance between each word of the original and the corresponding word of the target document image, and comparing the difference map and the Hausdorff distance to determine whether the corresponding words of the original and target document images are different; (f3) if the words of the original and target document images are not determined to be different in step (f2), identifying one or more candidate symbols in the word of the original document image and corresponding candidate symbols in the target document image; (f4) comparing image features of each candidate symbol of the original document image identified in step (f3) with image features of the corresponding candidate symbol of the target document image to determine whether any of the corresponding candidate symbols of the original and the target document images are different; (f5) if the corresponding symbols of the original and target document images are not determined to be different in step (f4), generating a difference map and calculating a Hausdorff distance between each candidate symbol of the original document image and the corresponding candidate symbol of the target document image, and comparing the difference map and the Hausdorff distance to determine whether any of the corresponding candidate symbols of the original and target document images are different; and (f6) if the corresponding symbols of the original and target document images are not determined to be different in step (f5), comparing shapes of each candidate symbol of the original document image and the corresponding candidate symbol of the target document image using a point matching method to determine whether any of the corresponding candidate symbols of the original and target document images are different; and (g) visualizing the differences detected in step (f).