How to Stop Adobe Reader DC’s Sidebar From Opening by Default?

Go to Edit > Preferences > Documents, and then select Remember current state of Tools pane.

Open a PDF document in Reader DC, and then collapse the Tools pane.

Click OK. Relaunch Acrobat Reader DC.

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.