1. An image processing apparatus comprising: a central processing unit (CPU) configured to act as: an obtaining unit that obtains a plurality of types of document images; a first classification unit that classifies each of the plurality of types of document images obtained by the obtaining unit into any of classification items included in one category among a plurality of categories, the plurality of categories each including a plurality of classification items; and a second classification unit that classifies, when there is a document image that is not classified into the one category by the first classification unit among the plurality of types of document images, the document image, which is not classified into the one category, into another category that includes at least one of classification items into which some of the document images are classified by the first classification unit; and a display panel that displays information regarding the classification of the plurality of types of document images, wherein when the one category is specified by a user, the first classification unit classifies each of the plurality of types of document images into the one category that is specified, when the one category is not specified by the user and is selected in accordance with a predetermined rule, the first classification unit classifies each of the plurality of types of document images into the one category that is selected, and when the plurality of types of document images are classified and the display panel displays information regarding the classification of the plurality of types of images on the display, the user is able to confirm the content of the plurality of types of document images that are classified and to process the plurality of types of images in accordance with a process that is determined for the specified or selected category.