10. The document data creating method of claim 9 , wherein the characteristic value is attached to every object included in the document data, and the characteristic value is calculated for every object included in the document data; and wherein, when it is determined that the reliability degree of the character included in a specific object is low, a value, which is derived from and different from the characteristic value of the specific object, calculated in the calculating step, is attached to the specific object as the digital signature to be included in the document data.
9. A document data creating method for creating document data that include a digital signature, comprising: recognizing a character based on image data inputted; determining a reliability degree of the character recognized in the recognizing step; calculating a characteristic value of the document data based on the document data; and attaching the characteristic value, serving as the digital signature, to the document data; wherein, when it is determined that the reliability degree of the character is low, a value, which is derived from and different from the characteristic value calculated in the calculating step, is attached to the document data as the digital signature to be included in the document data.