When you open read-only files in Emacs, you probably won’t want to attempt to save them. Emacs will warn you if you try and help you deal with it. However, most of the time, it is interruptive to your flow to deal with it—most of the time, we never want to modify read-only files.
In that case, View mode will make it easier for you:
Toggle View mode, a minor mode for viewing text but not editing it.
When View mode is enabled, commands that do not change the buffer contents are available as usual. Kill commands save text but do not delete it from the buffer. Most other commands beep and tell the user that the buffer is read-only.
Enable it automatically with
(setq view-read-only t)