Microsoft Office Compatibility Pack makes files created with Office 2007 and later work seamlessly on earlier versions of Office. If you're using an unpatched version of Office, this download will fix it. However, before you install this pack, make sure to download and install all of the latest updates from Microsoft.
If you're still using Office 2000, Office XP, or Office 2003, then this patch will help you easily open, edit, and save files created with newer versions of Office. It's a surprisingly big download -- checking in at 37MB. That means you should set aside some time to download it if you're on a slow connection. Once you install this pack, you don't have to do much to make it work. It automatically repairs the issue. It also adds support to the Word Viewer 2003, Excel Viewer 2003, and PowerPoint Viewer 2003 to open DOCX, XSLX, and PPTX files.
While it's surprising that Microsoft Office Compatibility pack is as big as it is, it's not loaded down with unnecessary extras, and it solves compatibility issues with older versions with little effort on your part.