Microsoft Word for Office 2003 and previous versions of Microsoft Word created documents in the .doc format. This changed at the premiere of the Microsoft Office 2007 version of Microsoft Word, where the native document format changed from .doc to .docx. While Microsoft Office 2007 can still open and save documents in the .doc format, .docx files are not compatible with previous versions of Microsoft Word and thus must be changed or converted before they can be opened in Microsoft 2003 or earlier Microsoft Office platforms.
Open the .docx format Word document in Microsoft Word 2007.
Change the extension by clicking the ribbon button at the top left of the toolbar and choosing "Save As." In the "Save as Type" drop-down menu that follows, select "Word 97-2003 Document (.doc)" as the format.
Open the newly saved version of the file in Microsoft Word 2003 or previous Word versions. Since not all fonts and other style dynamics are compatible from .docx files to .doc files, Microsoft Word might automatically change the font to the closest option available. If you do not like the font, you can select the text and choose a new font from the toolbar.
If you do not have Microsoft Word 2007, you won't be able to convert the .docx document using this version of Word. According to Microsoft Support, you must download and install the Microsoft Office Compatibility Pack for 2007 Office Word, Excel and PowerPoint File Formats. This is a free download and is the recommended compatibility converter for switching .docx to .doc format.