While Proton does not offer an official "portable" version (like a .zip or standalone .exe) of its desktop app, you can still achieve a portable experience using Progressive Web Apps (PWAs) or Third-Party Portable Clients with the Proton Mail Bridge. Method 1: The Browser-Based Portable App (Recommended)
Run Proton Mail Bridge (Note: The Bridge itself isn't officially portable, but you can copy the installed program files to a USB). proton mail desktop app portable
Cons:
Q: Can I just copy the installed Proton Mail folder from Program Files to a USB?
A: No. The app expects specific registry keys and user folders. It will crash or ask for re-installation. While Proton does not offer an official "portable"