1. A document processing apparatus comprising: an extracting unit that extracts text document information from a document data; an analyzing unit that analyzes a modification relation of a character string included in the text document information; an attribute unit that assigns an attribute indicating details of the modification relation to the character string, and embeds the attribute in the text document information; a document specifying unit that specifies a document-specifying character string that specifies other text document information, using the text document information in which the attribute is embedded by the attribute unit; a document-identification unit that assigns document identification information to the document-specifying character string, and embeds the document identification information in the text document information; a receiving unit that receives a character string; a determining unit that determines whether the text document information includes a document-specifying character string having the modification relation with the character string received by the receiving unit, based on the attribute and the document identification information embedded in the text document information; and an identifying unit that identifies other text document information indicated by the document-specifying character string, when it is determined that the text document information includes the document-specifying character string.