Starting in 2007, Microsoft Office introduced a new file format known as the Open XML file formats. These are documents with file extensions that ended in .docx (for Microsoft Word) or .xlsx (for Microsoft Excel).
But what if you are using Microsoft Office older than 2007 (such as Office 2003) where the file formats ends in .doc and .xls? Your Office 2003 can still open the new format documents provided that you get the free “Microsoft Office Compatibility Pack for Word, Excel, and PowerPoint File Formats“.
After installing that, every time you open a new format document in older Office, it will automatically do a file conversion with a quick dialog box that saying that file conversion is taking place.