1. A document management system, comprising: an image input unit to input a document as an electronic image; a character extraction unit to extract character information from the input electronic image; a word extraction unit to extract words from the character information; a document search unit to normalize the extracted words, to register the normalized words in an index, and to search electronic images using the index; an attribute information generation unit to generate attribute information of the input electronic image, where the attribute information includes the extracted words, positions and sizes of the extracted words in the input electronic image, and the normalized words referring to the positions and sizes of corresponding extracted words; a search condition input unit to input a search keyword for use by the document search unit when searching for a target electronic image; and a word highlighting unit to highlight the search keyword in the target electronic image found by the document search unit based on the attribute information.