welcome to the notepad++ community, @John-Dy-Chee-Puat
the -ro argument only works, if a file is not yet opened in a previous session.
the session has a higher priority and the read only setting of a session overrides the -ro switch.
if you want to open specific multiple files as read only, you can use the command line:
notepad++.exe -ro -nosession change.log license.txt.
note: this will disable session snapshot and periodic backup at the preferences, to try to avoid overwriting the previous session.xml.
recommended: if you usually open more or less the same bunch of files as read only, you can open those files, set them as read only, and save this as a session at file > save session for future use.
loading a saved session will set the saved read only status to the session’s state, regardless if the file is already opened in an opposite state.
best regards.