Eddy Office

Eddy Sheet for Excel files and Eddy Doc for Word files. Everything runs in your browser; your files never leave your computer.

Install on your laptop (Chrome or Edge)

Open Eddy Sheet above. Click the install icon at the right end of the address bar (a small screen with an arrow), or open the browser menu and choose Install Eddy Sheet (in Edge: Apps → Install this site as an app).
Do the same for Eddy Doc.
Both now have their own window and an icon in the Start menu, and they work without internet.

Right-click an .xlsx file → Open with → Eddy Sheet. For a .docx file choose Eddy Doc.
To make it the default for double-clicking, choose Choose another app, pick Eddy Sheet or Eddy Doc and tick Always use this app.
The first time, the browser asks whether the app may open that kind of file. Choose Allow.
Click Save to write your changes back into the same file. The first save asks once for permission to change the file. Save a copy downloads a separate copy instead.

Android: in Chrome, tap ⋮ → Install app. Then in WhatsApp, Gmail or Files, tap Share on an Excel or Word file and choose Eddy Sheet or Eddy Doc.
iPhone / iPad: in Safari, tap Share → Add to Home Screen. Open files with the Open file button (it browses the Files app and iCloud Drive).
On phones and tablets, Save downloads a new copy instead of changing the original file.

If a file is also open in Excel or Word, Windows locks it; Eddy then saves a copy to your downloads folder instead.