.doc v. .docx

Versions of Microsoft Word up to and including Word 2003 saved files with a ‘.doc’ extension. From Word 2007 onwards, by default, documents end in ‘.docx’. For most users, most of the time, this is an esoteric, inconsequential distinction… until you’re unable to open the newer file format using an older version of Office. .doc is a binary file format. .docx files are actually compressed ZIP archives containing XML data. Analogous format changes were made for Excel spreadsheets and PowerPoint presentations.

• .docx files store text, pictures, and formatting separately, so they can be more easily recovered in the event of data corruption. This property also makes it substantially easier to replace graphics or remove sensitive metadata from large batches of documents.
• .docx files are compressed, making them smaller and easier to send as e-mail attachments than .doc files.
• .docx files can’t contain macros, so they are incapable of executing malicious code.
• Because XML is an open format, documents or their properties can be edited without Microsoft products, and data can be easily exchanged with enterprise business software.

Microsoft offers a free Compatibility Pack that enables users of Office 2000, Office XP, and Office 2003 to open, edit, and save files in the new Open XML formats.

Microsoft on XML file formats