To protect a Windows Notepad file as Read-Only you can follow the below steps.
Step 1: Create your file using Notepad and save it.
Step 2: Now right click on the file and select Properties.

Step 3: Under General tab, under Attributes, check the option "Read-only"

Click OK. Now no one will be able to edit and make changes to the file.
Simply uncheck "Read-Only" option to make this file editable again.
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