Adobe’s latest version of Acrobat is called Adobe Acrobat DC, with DC standing for Document Cloud. Adobe tries to push their cloud features to customers. Some of their new tools are available on the right pane within the program (called the Tools Pane).
However, if you mostly use Adobe Acrobat DC to simply read document (without using any of their tools), the sidebar/pane on the right is annoying because it takes up screen space.
To hide the right pane, you should head to settings and change it there.
- Edit > Preferences (or use CTRL + K)
- In the left menu, pick Documents
- Uncheck Open tools pane for each document
The same should apply for Windows and Mac OS X.