1. An image processing apparatus comprising: a processor configured to execute: an extracting unit configured to extract identification information items from image information items of a document group that includes a plurality of individual documents, each of the identification information items being included in a corresponding one of the individual documents; a corresponding information generating unit configured to generate corresponding information items that indicate corresponding relationships between the identification information items and the image information items of the individual documents; an operation mode selection unit configured to select, as an operation mode of the corresponding information generating unit, one of a first mode in which one of the corresponding information items that corresponds to a predetermined one of the image information items is generated, and a second mode in which, when the extracting unit has extracted the identification information items from the predetermined areas of the image information items, the corresponding information items that correspond to the image information items are generated; and an electronic document generating unit configured to rearrange an order of the identification information items on a basis of contents of the identification information items and generate an electronic document that includes the image information items of the document group, the corresponding information items, and the identification information items that have been sorted, wherein in the first mode, only individual documents among the plurality of individual documents having the corresponding information items match the predetermined one of the image information items are designated to be rearranged by the electronic document generating unit, while the remaining individual documents are arranged in the order of the scan with respect to the rearranged individual documents, or wherein in the second mode, only individual documents among the plurality of individual documents having the identification information are bookmarked, while the remaining individual documents are designated to be rearranged by the electronic document generating unit, while the remaining individual documents are arranged in the order of the scan with respect to the rearranged individual documents.