When a file is open, it displays in a graphics window by default. If more than one file is open, each file appears in its own graphics window. You can open the same document in multiple windows.
The window you are working in is the active window. Click with your cursor inside any displayed graphics window to make it the active window.
Alternatively, to activate a window you want to work on, on the ribbon, View tab, Windows panel, on the Switch drop-down list, select the file. When more than nine files are open, the More Windows option appears at the bottom of the Switch drop-down list. Click More Windows to open the Select Window dialog box, where you can activate another file.
You can control the visibility of the tabs at the bottom of the window that bear the names of documents that are open. On the ribbon View tab, Windows panel, in the User Interface drop-down list, select, or clear the check box for Document Tabs.