Most users may have found this problem: Microsoft Word 2007 is opened with 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. As a matter of fact, many Word users are used to the classical Word 2003, so they usually save Word 2007 to Word 2003 or Word 2010 to Word 2003. Sometimes, you send a .docx file to your friend but he or she can't view .docx with Word 2003, so you need to convert docx to doc so that he or she would read the .docx file.
And it's very easy to convert docx to doc format if you really want to do this, no need third-party software, just save Word 2007 to Word 2003 or convert Word 2010 to Word 2003 directly as below tips.
Step 1, open the Word file.
Step 2, press the Office button on top left, go to Save As and choose Word 97-2003 Document.
Step 3, enter a new name at the File Name field or just use the same name as original .docx file.
Step 4, click Save to convert docx to doc format, in seconds, the file will be viewed in .doc format as soon as the conversion from Word 2007 to Word 2003 or Word 2010 to Word 2003 is completed.
As I am using Office 2007, so, this is the normal process to save Word 2007 to Word 2003, to convert Word 2010 to Word 2003 is a bit different. From step 2, you need to click to Save & Send > Go to Change File Type and choose Word 97-2003 Document as below to convert Docx to Doc.
And if you want to convert between different Office formats, like Word and PowerPoint, you can read articles: how to convert Word to PowerPoint? Of course, you can also convert doc to PDF for sharing with colleagues or other people and convert PDF back to Word to make changes for original PDF.