When you upload a document, or publish a document from the Unpublished Documents sub-node, you become its owner and have full access to it. You can grant other users access to it.
If you have granted folder-level permission to other users and allowed the permission to apply to the contents of the folder, those permissions will apply to the documents that you add to it automatically. They will appear as selected in the document permissions window. However, you can modify these permissions per document if needed.