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, wherein 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, the output process control section is 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, when a target feature point is one of feature points, which are information indicative of positions of centroids of connected components between pixels on the image, the feature is calculated with respect to each feature point by calculating a feature of the target feature point in accordance with a positional relation between peripheral feature points located around the target feature point, the image matching section determines the similarity in accordance with a matching degree of features and position information of feature points at which the features on input image data match, and when an image of the input image data is similar to an image of the reference document, the output process control section controls at least the output process of an image located on a position specified by the position information of the feature points at which the features match.