A client recently sent me a word document with a list of attendees for an upcoming course. I was surprised to see the document was a .doc as this file format has not been in use since Office 2003.
In Office 2007 and later versions Microsoft changed the internal file format of MS Office files to implement new features and make them more secure. The new file formats mostly added an x to the previous extension. So .doc became .docx.
If you still have a few files in the older format lingering on your system, my suggestion would be to convert them unless you have specific business reasons to keep them in the old format.
If you see [Compatibility Mode] after the file name in the title bar, when you open the File tab you’ll see a Convert button at the top of the list. This provides an easy way to convert the file to the latest format as it saves it in the new format and then deletes the old file so you only have one version.
Note, too, that Microsoft made small changes in the internal format in some of the recent versions so if you open a .docx file created in Word 2007 in, say, Word 2016, you’ll see [Compatibility Mode] in the title bar and have a Convert button on the File menu.