1. A binarization device for payment or accounting documents including sensitive data, wherein said documents are represented by image files having significant pixels associated with the sensitive data and background pixels and wherein the sensitive data are located in a respective data window on a document of a given type of forms and can be constituted by alphanumeric characters, said binarization device comprising primary binarization means for obtaining a primary binarized document file of said document and further comprising: memory means for storing, identification files including identifying images and location information associated to respective given types of forms and the localization of the data window with respect to said given types of forms; window identification means for identifying and localizing in the image file of said document a window file including the significant pixels of said document, as comparison with the identification files of the memory means; background estimation means for estimating the contribution of the background among the pixels of said window file; window background subtracting means for subtracting the contribution of the background from the window file, whereby forming a background free window file having significant pixels in evidence with respect to residual background pixels; window file binarization means for binarizing the background free window file with formation of a background free binarized window file; filtering means for filtering spurious pixels from the background free binarized window file, forming a binarized window file; merging means for providing a document binarized image file of the document as merging of the primary binarized document file and the binarized window file to provide the binarized window file in the data window; and wherein the background estimation means comprises sequencing means for a sequential analysis on groups of pixels, by applying morphological expansion operators on each group of pixels and following erosion of said group of pixels for estimating the contribution of the background of the window to be subtracted from the pixels of the data window.