1. A document management system, comprising: an image input unit to input documents as electronic images; a memory to store the input electronic images; a character extraction unit to extract character information from the input electronic images; a word extraction unit to extract words from the character information; a search condition input unit to input a search keyword; a document search unit to normalize the extracted words and to register the normalized words in an index, the document search unit being also configured to normalize the search keyword input by the search condition input unit and to find a target electronic image by searching the index with the normalized search keyword; an attribute information generation unit to generate attribute information of the respective input electronic images, where the attribute information includes the extracted words, positions and sizes of the extracted words in the corresponding input electronic images, and the normalized words referring to the positions and sizes of the corresponding extracted words; and a word highlighting unit to highlight a matching normalized word corresponding to the normalized search keyword and/or a matching extracted word corresponding to the matching normalized word in the target electronic image found by the document search unit based on the attribute information of the target electronic image.