You can use an application to convert a DOCX file to DOC file format. Convert DOCX to DOC, for example, allows you to perform this conversion or you can use an online service. For this purpose, Google can give you plenty of results, however, you can also use a simple trick which involves uploading the document to GDrive. If you have a Gmail account, visit GDrive and upload the file then open it in its editor. It supports both DOCX and DOC formats. Open the file then use Save and Download and select DOC instead of DOCX. The conversion will take place automatically on their servers.