You can make a document private from the share panel. Please follow the instructions mentioned below,
- Open the document you would like to change to private.
- Click on the Share button on the top toolbar
- Select Specific Users (or any other option than ‘Public’) option on the share panel
- Close the share panel
Your document is now private and will not be able to access publicly by anyone.
Make a Document Private
Find out how to share a Document in a public channel: How to change a Document to Public?