By default, files and documents are private to the user who created or uploaded it. To change an item from private to public, click on the lock icon and confirm in the pop up to change it to public . This will make the document or file available to all project members with view access in documents. Public items will not have the option to Share with members since it is already available for viewing by project members.
Users will need view and edit permissions to mark entities as public or private. Depending on the settings of a user's project permissions, they may be able to view all document entities, or just a subset of them. Entities specifically shared to a member will be viewable to them. Folders are public entities, and this cannot be changed.