Go to Edit > Preferences > Documents, and then select Remember current state of Tools pane.
Close the Tools pane in Acrobat Reader
Beginning with the July 2015 Acrobat release, you can control the Tools pane behavior with a preference.
To permanently hide the RHP in Acrobat Reader, do the following:
Open a PDF document in Reader, and then collapse the Tools pane.
Click OK. Relaunch Acrobat Reader.
Acrobat Reader now remembers the state of your Tools pane. If it was collapsed (or expanded) during the last launch, it remains collapsed (or expanded) when you launch again.
Note: You can get the Acrobat Reader update by choosing Help > Check for Updates or by downloading the installer.