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 items, so documents will always open in read‑only mode for them by default. For everyone else (editors, owners, and contributors) who has edit rights, the document will always open in edit mode. Therefore, the only option is to apply this setting individually at the document level.

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

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