The advent of Microsoft Word 2007 meant a new file format: .docx. This led to some confusion because Microsoft Word 2007 uses .docx as its default format. This difference is not necessarily clear until a Word user with a previous version tries and fails to open the .docx file.
Open the .docx file in question by double-clicking on the file icon. If you're saving a file that is already open in Microsoft Word, go directly to Step 2.
Click the round icon in the upper left corner of your screen. Highlight the "Save As:" option by hovering your mouse over it, then select "Word 97-2003 Document" from the list to the right.
In the next window that opens click "Save." You might want to consider renaming the file before doing this, as having .doc and .docx files under the same file name could be very confusing. Do this by typing a new name into the "File Name:" portion of the window before clicking "Save."
Users running previously versions of Microsoft Word can download a compatibility pack to help them handle .docx files, but if you save the files in .doc format, you won't have to bother. .Docx was introduced to improve file security and manageability. .Doc files have become a magnet for virus writers and cannot be modified one component at a time, as .docx files can.