Since the version 2015.009.20069, multiple documents are opened as Tabs instead of Windows of Acrobat.
You can use Ctrl+Tab to switch to next tab and Ctrl+Shift+Tab to switch to previous tab.
來源：https://community.adobe.com/t5/acrobat-discussions/move-between-documents-keyboard-shortcut-acrobat-pro-dc/td-p/7703541
全部的keyboard shortcuts: https://helpx.adobe.com/acrobat/using/keyboard-shortcuts.html