You can get the contents of a specific document in the repository by using its id. The format of the URl and the parameters that you can use are detailed in the service document.
Here is an example of a URL to retrieve the contents of a specific document in a Content Services on-premise instance:
https://localhost:8080/alfresco/api/-default-/public/cmis/versions/1.1/atom/content?id=824ba7cd-dcee-4908-8917-7b6ac0611c97
The response body is the content of the document. The format is specific to the type of content, so for example, getting the contents of a text document returns a text response body.