By default, the Select tool is active when Reader opens, because it is the most versatile tool.
Specialized tools, such as those for zooming in or adding review comments, are available in toolbars and in the Tools menus.
Select a tool in a toolbar.
Choose Tools > [toolbar name] > [tool].
You can use these tools temporarily, without deselecting the current tool.
When you release the keys, Reader reverts to the previously active tool.