Some editors lets you save files as Read-Only so it cannot be modified if a file was set read-only using notepad++, the can be made editable by opening the file and going under Edit → Set Read-Only (Uncheck). If a file in the file-system is marked as read-only, it can be made editable by clearing the read-only flag, This can be done by selecting Edit → Clear Read-Only Flag.
If you want to save a file as Read-Only then go to Menu → Edit → Set Read-Only (Check) and save the file.

Notpad-plus-plus-readonly-file
✌️ Hint: If the file is of type Read-Only then you will see that the Save icon for the file will be gray in color (instead of red → unsaved or blue → saved)
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