1. An image data output processing apparatus for performing an output process of input image data, comprising: an image matching section for extracting features of an image from input image data and comparing the extracted features with features of an image of a preliminarily stored reference document so as to determine a similarity between the image of the input image data and the image of the reference document; and an output process control section for controlling an output process of the input image data in accordance with a result of determination by the image matching section, when the input image data is indicative of an N-up document in which images of a plurality of documents are assigned, the image matching section determining a similarity between each image of the N-up document and the reference document, and the output process control section being capable of controlling the output process of each image in accordance with a result of determining the similarity between each image of the N-up document and the reference document.