This is the gold standard. It is 100% free, officially supported as a portable app, and handles old Office 2003 files (and new .docx files) perfectly. Where to get it: PortableApps.com
Once downloaded, the portable edition can be run from a portable device by: Microsoft Office 2003 Portable Download WORK
Microsoft states: “Unsupported versions of Office do not receive security updates. Using them on internet-connected devices creates significant risk. We strongly recommend upgrading to Microsoft 365 or Office 2021.” This is the gold standard
While it offers a nostalgic and lightweight experience for legacy systems, there are critical modern considerations regarding its use today. Key Features officially supported as a portable app