If you want to protect a document without having to create a protection, you can do this by dragging the document to "Custom". You can also double-click on this "Custom protection" tile and select a document.
Configure the users and their permissions. A user is identified by their email address. Accept the configuration and ... the document is protected!
You have the option of setting an expiration date for the document or allowing users offline access to the document. You can also add a watermark (only for PDF), which will appear on documents for users with read-only permissions.