To open an extra window on an already open document:
Click on an already open document window, to make it the active window.
Choose menu option Window > New Document Window (or keyboard shortcut ⇧⌘N Shift-Command-N).
You can open multiple documents at the same time. Each document is displayed in its own document window.
Each document can also be opened in multiple document windows, with each document window having it's own independent view settings, enabling you to work on different parts of a document at the same time, in different windows and at different zoom levels, etc. Edits that you make in one document window are displayed in all visible views, in real time.
Click on an already open document window, to make it the active window.
Choose menu option Window > New Document Window (or keyboard shortcut ⇧⌘N Shift-Command-N).