This is the last version that officially supports OS X 10.7. It is free, open-source, and opens all Word/Excel files perfectly. Apple iWork '09:
If you only need to read and lightly edit Office files, consider dropping Microsoft Office entirely: Microsoft Office For Mac Os X Lion 10.7.5
If you cannot source a copy of Office 2011, consider these alternatives: This is the last version that officially supports OS X 10