1. A document processing apparatus comprising: an extracting unit that extracts text document data from first document data contained in a first file; an analyzing unit that identifies a modification relation between a first character string and a second character string included in the text document data; an attribute embedding unit that embeds an attribute in the text document data, the attribute representing the modification relation; a document specifying unit that identifies the second character string as being a document-specifying character string that specifies second document data contained in a second file differing from the first file; a document-identification unit that embeds a document tag in the text document data, the document tag tagging the second character string as the document-specifying character string; a receiving unit that receives an input character string; a determining unit that determines that the input character string matches the first character string, and, in response to determination that the input character string matches the first character string, identifies the document-specifying character string having the modification relation with the first character string based on the attribute and the document tag embedded in the text document data; an identifying unit that identifies the second document data contained in the second file specified by the document-specifying character string in response to the determination that the input character string matches the first character string; and a central processing unit configured to execute at least the determining unit.