In case your working with Microsoft Office 2007 you probably know that Office 2007 uses another extensions in this version like .docx for Word! Each time when you save a file it wouldn’t open in any previous version of Office! To resolve this problem, microsoft provides Office Compatibility Pack which enables users to open,edit and save files using the file formats new to Word, Excel, PowerPoint 2007. It can also be used with MS Office Word Viewer 2003, Excel Viewer 2003, and PowerPoint Viewer 2003.

Download: Office Compatibility Pack
Source: IT Tech Talk