10. A document processing method comprising: extracting text document data from first document data of a first file; analyzing a modification relation between a first character string and a second character string included in the text document data; embedding an attribute in the text document data, the attribute indicating the modification relation; identifying the second character string as being a document-specifying character string that specifies second document data of a second file differing from the first file; embedding a document tag in the text document data, the document tag identifying the document-specifying character string; receiving an input character string; determining that the input character string matches the first character string in the text document data; in response to the determining, identifying the document-specifying character string having the modification relation with the first character string based on the attribute and the document tag; and identifying, in response to the identifying the document-specifying character string, the second document data represented by the document-specifying character string.
10. A document processing method comprising: extracting text document data from first document data of a first file; analyzing a modification relation between a first character string and a second character string included in the text document data; embedding an attribute in the text document data, the attribute indicating the modification relation; identifying the second character string as being a document-specifying character string that specifies second document data of a second file differing from the first file; embedding a document tag in the text document data, the document tag identifying the document-specifying character string; receiving an input character string; determining that the input character string matches the first character string in the text document data; in response to the determining, identifying the document-specifying character string having the modification relation with the first character string based on the attribute and the document tag; and identifying, in response to the identifying the document-specifying character string, the second document data represented by the document-specifying character string. 14. The method according to claim 10 , further comprising: displaying, as a candidate, a candidate character string associated with the attribute; receiving a selection of the candidate character string; and searching for the candidate character string in the text document data.