If you use Adobe Reader offten you will notice a tool pane on the left which you don’t use you can easily hide it by following this tutorial.
1. First open adobe reader and you will see the tool pane which you can easily hide it by pressing Shift+F4 , but if you want it to not open by default at startup , after you have hide it click on Edit and Preferences.
2. Then click on Documents tab and make sure Remember current state of Tools Pane option is checked/ticked , and click Ok.
Now restart the application to see the changes.