What happens when a user opens a .docx file in Word 2003?

When a user opens a .docx file in Word 2003, the most likely outcome is that the file may cause compatibility issues. The .docx format is part of the Microsoft Office Open XML document format introduced in Microsoft Word 2007, while Word 2003 uses the earlier .doc format. Since Word 2003 does not natively support .docx files, it may struggle to interpret some of the newer features, formatting, or elements that were not available in earlier versions.

This can lead to various problems, such as missing formatting, loss of data, or certain features not displaying correctly. Users might also receive an error message indicating that the file format is not supported or that a converter is needed. While it's possible to install a compatibility pack to enable Word 2003 to open .docx files, without it, opening the file may not work as expected.