1. An image processing device comprising: at least one processor and memory, cooperating to function as: an input unit configured to input document image data; a region division unit configured to divide the document image data into a plurality of regions according to attributes, the divided regions including a text region, a caption region and an object region which is accompanied by the caption region; a character recognition unit configured to obtain character information by executing a character recognition process for each character within each of the text region and the caption region divided by said region division unit; an anchor expression extraction unit configured to extract, from the character information in the caption region, an anchor expression which includes a predetermined character string identifying the object region; a text search unit configured to search for the anchor expression extracted by said anchor expression extraction unit from the character information in the text region; a link information generation unit configured to generate two-way link information associating an anchor expression peripheral region and an image peripheral region with each other, the anchor expression peripheral region being a region including the anchor expression found by said text search unit, and the image peripheral region being a region including the object region, wherein, in a case where said text search unit finds one anchor expression from the character information in the text region, said link information generation unit generates the two-way link information associating the image peripheral region with one anchor expression peripheral region which is a region including the one anchor expression found by said text search unit, wherein, in a case where said text search unit finds a plurality of anchor expressions from the character information in the text region, said link information generation unit generates the two-way link information associating the image peripheral region with a plurality of anchor expression peripheral regions which are regions including the plurality of the anchor expressions found by said text search unit; and a format conversion unit configured to generate electronic document data including document image data and the two-way link information, wherein, in the case where said text search unit finds the one anchor expression from the character information in the text region, the two-way link information includes a first link and a second link, the first link including information for displaying the associated one anchor expression peripheral region when a reader of the electronic document takes a predetermined action on the image peripheral region, and the second link including information for displaying the associated image peripheral region when a reader of the electronic document takes a predetermined action on the one anchor expression peripheral region, and wherein, in the case where said text search unit finds the plurality of anchor expressions from the character information in the text region, the two-way link information includes a third link and a fourth link, the third link including information for displaying information about the plurality of the anchor expression peripheral regions as a plurality of candidates of link destinations from the image peripheral region when a reader of the electronic document takes a predetermined action on the image peripheral region, and the fourth link including information for displaying the associated image peripheral region when a reader of the electronic document takes a predetermined action on any one of the plurality of the anchor expression peripheral regions.
6. The image processing device of claim 1 , wherein the third link includes information for displaying a list about the plurality of the anchor expression peripheral regions as the plurality of candidates of link destinations from the image peripheral region when the reader of the electronic document takes the predetermined action on the image peripheral region, so that the reader can select which of the anchor expression peripheral regions is displayed.