Annoyingly, the default setting for Adobe Reader is to open multiple pdf documents in different tabs of the same window, rather than in multiple windows. This makes it impossible to compare two documents side by side. However, you can easily fix this in the preferences:
- Open Adobe Reader and go to Edit > Preferences > General.
- Some way down the list of options, there’s one that says ‘Open documents as new tabs in the same window (requires restart)’, which you need to untick.
- Restart Adobe Reader, and you should now be able to open each pdf in a separate window.