Microsoft introduced new format with Office 2007. Word 2007 saves file as .docx. Excel 2007 saves file as .xlsx and so on. But most of the world is still using old versions of Office. These people might have trouble accessing your files. So, for the time being it might still make sense to save files in old doc or xls format.
Saving Excel 2007 files in XLS format
Click Office button and click Excel Options at the bottom right corner. Go to save option and change the default save option to Excel 97-2003 Workbook (*.xls). Now on files will be saved in old format by default.
Saving Word 2007 files in DOC format
Click Office button and click Word Options at the bottom right corner. Go to save option and change the default save option to Word 97-2003 Document (*.doc). Now on files will be saved in old format by default.
