As said, the file is corrupted. a Office .docx file is actually a zip file, and inside a lot of information is stored via XML. some of the XML data is corrupted. The error is coming from the XML parser.
Technically you could copy it, rename the copy to .zip, extract it, and then try to change the file to fix it and zip up the new files and rename it back to docx, but if it has one problem it probably has innumerable other issues.