Microsoft added new file formats to Microsoft Word, Excel, and PowerPoint 2007, 2010, 2013 and 2016 to reduce file size, improve security and reliability, and enhance integration with external sources. To help ensure that you can exchange documents between Microsoft Office releases, Microsoft developed a Compatibility Pack for the Word, Excel, and PowerPoint file formats. This Compatibility Pack works with files created by Office 2013, Office 2010 and the 2007 Office systems.
The Open XML Formats usher in a new era of openness and transparency for Word, Excel and PowerPoint. Solution developers can take advantage of a host of new integration opportunities to connect documents to important sources of information. Learn more about the Open XML Formats
By installing the Compatibility Pack along-side of Microsoft Office XP, or Office 2003, you will be able to open, edit, save, and create files using the formats that are used by Office 2016, Office 2013, Office 2010 and the 2007 Office systems.
OpenXMLDeveloper.org is a free community to discuss and exchange ideas regarding the development of solutions for the Open XML Formats.
Learn about XML in Microsoft Office on MSDN. At this site, you will find technical articles, code samples, developer documentation, and multimedia presentations on working with XML in Microsoft Office.