1. A procedure for verifying the integrity of documents, in which the integrity of a document to be verified is verified with respect to an original document, characterized in that it comprises a previous characterization stage of the original document, which in turn comprises: in case the original document is a printed document, obtaining an original digital document through digitalization; representing the original digital document in a matrix format; adapting the document in matrix format to a determined A×B resolution; dividing the document into regions having an N×M pixels size; obtaining coefficients in the transform domain through applying to each one of the regions, a transform; quantifying said coefficients; selecting, for each region, already quantified coefficients; obtaining a hash of the original digital document from the selected coefficients for each region; and characterized in that on the document to be verified, a stage of integrity verification is carried out, which in turn comprises: in case the document to be verified is a printed document, obtaining a digital document to be verified through digitalization; representing the digital document to be verified in a matrix format; obtaining a corrected image, taking the following steps in any order: adapting said document in matrix format to the same resolution A×B determined to obtain the hash of the digital original document; correcting the inclination of said document in matrix format; obtaining, from the corrected image and the hash of the digital original document, some first displacement coordinates to compensate the displacement produced between the location of the contents in the original digital document and the location of the contents in the digitalized document to be verified; obtaining some optimal displacement coordinates for each one of the regions of the corrected image, using regions of N×M pixels obtained in the same way as in the characterization stage, taking the following steps for each region: selecting a region of the corrected image taking into account the displacements produced according to the previously obtained first coordinates of displacement; obtaining the coefficients in the transform domain by applying, to said selected region, the same transform used in the characterization stage of the original document, quantifying said coefficients; obtaining said optimal coordinates of displacement from the comparison of said quantified coefficient with the coefficients of the corresponding region of the hash; obtaining at least one metric from the quantified coefficients of the corrected image and the coefficients corresponding to this region in the original image extracted from the hash of the original document, taking into account the optimal displacement coordinates for each region of the corrected image; deciding on the integrity of each region of the digitalized document using at least one of the previously calculated metrics and comparing it with a determined threshold; and deciding on the integrity of the document based on the results of the previous step.