How can I add a file/quiz on moodle which I can make invisible to students and can make it visible whenever I want to make it.
I'm using the latest version of Moodle
You can use the Restrict access settings. Also, a totally manual way of doing it is by selecting the option "Make it available"/"Make it unavailable" of the menu Edit next to each activity or resource.
Sorry, I meant "Show/Hide" options, not "Make it available"/"Make it unavailable".