Microsoft Office 2007 (Windows) and 2008 (Mac) use a new set of file formats (.docx, .xlsx, .pptx) that are not compatible with older MS Office versions.
MicroSoft offers converters and viewers. Mac converters and viewers are listed at the bottom of the page with a separate link.
Office 2007 | 2008 users can use Save As to save files in 97-2003 formats (.doc, .xls., .ppt) so users with previous MS Office versions can open the documents. The Options/Preferences setting has a Save As feature to set the default to the older format.
Although most campus lab computers and many campus office computers have the new versions installed, students and faculty may not have the new versions on home computers. Using the Save As for 97-2003 formats will avoid having files that won't open on those computers.
Other applications, such as OpenOffice for both Windows and Mac or NeoOffice for Mac will Open and Save As the new formats.