By default, opening a PDF in Adobe Acrobat Reader DC also displays the Tools Pane, which can be annoying for some. While it is not possible to completely remove the toolbar, you can collapse it, but this gets reset each time some other document is opened. This behaviour can be turned off so the pane remains collapsed when opening other documents.

The setting is found in the Preferences:

1. in the menu click on Edit → Preferences, or press CTRL + K.
2. In the Categories list, click on Documents.
3. Unselect Open tools pane for each document, then click OK.

The very next time you open a document, the pane may still be visible if you haven't hidden it during the previous run, but if you collapse it now, subsequently opening another document will not re-open the pane.

This also works for Reader DC on Mac. Note that early versions of DC will not have this checkbox. In that case, please update to the latest version.