Open Files from SharePoint in "Protected" mode by default

SharePoint and Office Online do not provide a built‑in way to force all documents in a library (or for specific users) to open in read‑only mode by default while still allowing editing when needed.

Readers cannot edit the items; for them, the document will always open in read‑only mode by default. Otherwise (for editors, owners, and contributors), you need to set this property at each document level individually.

I believe you want the document to always open in read‑only mode, regardless of the user’s permissions. I’ve already provided an answer to this question; please refer to that response.

 Sharepoint - Prevent a new version each time a document is read | Microsoft Community Hub

If my post solved your issue or answered your query, please mark it as a Solution and give it a Like.