This blog will help you to identify why documents are opening in the Read-Only mode in a particular library from the SharePoint Application.
We have all come across this kind of issue once or twice; i.e., whenever a user is trying to open a document from the library, it opens in read-only mode.
Whenever this kind of issue occurs, the first symptom would be that it will prompt for the login when the user tries to open the document in a client application and then, it will open the file in read-only mode. So, the easiest way to find whether a document library is converted into the read-only mode is to try and sync that library with your local machine via OneDrive for Business.
In my case, I have reproduced the read-only issue in my demo library and all documents are opening in read-only mode. So, I am going to find whether my document library is changed into the Read-only mode or not.
I am just clicking the sync option to enable the synchronization with SharePoint library to my local system.

It will prompt you to get access to your local system; just click "Allow".
In the below picture, you can see the highlighted text. “The site is read-only. You can’t make any changes to file or folder on this site”. Once you identify this message, you can cancel the synchronization.
This means my library is converted in read-only mode.
Here are some possibilities for this occurrence.
- The user may create some extra columns where the content type is associated with a document library.
- User may upload the documents in a bulk way like drag and drop, so the required columns may not be filled.
- Some workflow may cause this problem.
These are the issues I have faced in my past. If your issue is not related to this, then you can get some more ideas on the internet.

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