To add annotations to a page, activate annotation mode by clicking the middle segment of the [View Mode] button in the toolbar. This will show the annotations bar at the top of the preview canvas.
The left half of the annotations bar is used to select the type of annotation you want to add. The right half can be used to alter annotation properties, like color, line width and font. The button on the far right of the bar shows a list of all annotations in the document.