1. An image processing apparatus comprising: an extracting unit that extracts 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 that generates corresponding information items that indicate corresponding relationships between the identification information items and the image information items of the individual documents; and an electronic document generating unit that rearranges an order of the identification information items on a basis of contents of the identification information items and generates 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.