1. A method comprising: scanning a page of a paper document to generate a first digital page image, the first digital page image formed of first pixels; obtaining a second digital page image which corresponds to a second page, the second digital page image formed of second pixels; comparing the first digital page image pixel-by-pixel with the second digital page image to generate a score that indicates a degree to which the first digital page image differs from the second digital page image; and applying a brightness normalization algorithm which includes: calculating a sum of values of the first pixels; calculating a sum of values of the second pixels; comparing said sums to generate a pixel value scaling factor; and applying the pixel value scaling factor to all of the first pixels or to all of the second pixels; said brightness normalization algorithm being applied before said comparing step.