1. Right click at the blank area of the toolbar within the PDF -> Customize Toolbar...;
2. Or click View on main menu -> Customize Toolbar..., you can choose the most frequently-used tools and drag them to the toolbar;
3. Or click Tools on the toolbar and select Customize Toolbar ; There, you can select Add icons to the toolbar or Remove them. Tap Reset, and the default one will show. You can drag the icons to change the location or use Space to define each section as you like.