PS: this was one of the key reasons back in the day that I moved away from Google Docs...
If Proton wants to be serious about creating an alternative to Google and Microsoft - ie critically an environment that does not deliberately tie you in and respects user privacy and data ownership (which implies transferability) they should make the ability to store and work on Open Document Format a starting point....

A key decision element for me to consider migration from MS Office is to move to a platform that natively supports an open document format - so that editing is feasible whether working natively in Proton web or on a document synchronised to a linux desktop using Open Office or using mobile.
The lessons from the last 18 months is that I never want to be tied into a Tech Bro environment again - which means my files are mine, and I am happy to rent space and functionality in a cloud environment...

This is already implemented for Sheets so it has been edited to just refer to docs. Thanks.

I can't understand why OPF is not supported in the slightest by Proton ecosystem. Not just that any person concerned with privacy is going to prefer it, but also that being fully open source and designed for portability, sound likely easy to integrate.

Duplicate, and not currently possible to implement per this thread: https://protonmail.uservoice.com/forums/932839-proton-drive/suggestions/50778653-preview-and-open-odt-opendocument-text-files