2. A document data creating apparatus for creating document data that include a digital signature, comprising: a character recognizing section to recognize a character based on image data inputted; a reliability determining section to determine a reliability degree of the character recognized by the character recognizing section; a characteristic value calculating section to calculate a characteristic value of the document data based on the document data; and a digital signature section to attach the characteristic value, serving as the digital signature, to the document data; wherein, when the reliability determining section determines that the reliability degree of the character is low, the digital signature section attaches a value, which is derived from and different from the characteristic value calculated by the characteristic value calculating section, to the document data as the digital signature to be included in the document data.
2. A document data creating apparatus for creating document data that include a digital signature, comprising: a character recognizing section to recognize a character based on image data inputted; a reliability determining section to determine a reliability degree of the character recognized by the character recognizing section; a characteristic value calculating section to calculate a characteristic value of the document data based on the document data; and a digital signature section to attach the characteristic value, serving as the digital signature, to the document data; wherein, when the reliability determining section determines that the reliability degree of the character is low, the digital signature section attaches a value, which is derived from and different from the characteristic value calculated by the characteristic value calculating section, to the document data as the digital signature to be included in the document data. 3. The document data creating apparatus of claim 2 , wherein the characteristic value is attached to every object included in the document data, and the characteristic value calculating section calculates the characteristic value for every object included in the document data; and wherein, when the reliability determining section determines that the reliability degree of the character included in a specific object is low, the digital signature section attaches a value, which is derived from and different from the characteristic value of the specific object, calculated by the characteristic value calculating section, to the specific object as the digital signature to be included in the document data.