1. A document processing device comprising: a document data acquiring part for acquiring document data; a character string extracting part for extracting character strings that correspond to at least one of a heading, a title, or a subtitle as bookmark candidate character strings from said document data acquired by said document data acquiring part; a format creating part for deriving respective features of said bookmark candidate character strings extracted by said character string extracting part, categorizing said bookmark candidate character strings having a common feature into one group, and creating a format for the group based on the common feature of the group; a display part on which said bookmark candidate character strings extracted by said character string extracting part are displayed in a list form, and on which the format corresponding to said bookmark candidate character strings created by said format creating part is displayed; and a format correcting part for receiving a correcting operation of a user given to the format displayed on said display part and correcting the format in response to the correcting operation, wherein said character string extracting part extracts the character strings again that have the common feature of the format corrected by said format correcting part from said document data as said bookmark candidate character strings.