With the Insert Freehand Line tool (F), also called the Pen tool, you'll be able to draw any line type you wish. This tool is made to simulate the behavior of a real pen, so it is hard to get a pure straight line or nice curve with it. You will use it when the final result doesn't need to be very precise. It can be used if you like to give a hand-made effect. We sometimes use it to annotate: it happens when I have to report or correct things on a layout.
Go back to a document we've already made. Open the Layers window from the Windows menu.
Create a layer by clicking on the + button at the bottom left-hand of the layer window. Double-click on its name and enter Comments.
Deactivate printing for this layer by deselecting the second box on its line.
Now activate the Insert Freehand Line tool (F)
Sketching with this tool is really nice and fast: press the left mouse button and drag. Release when you want to stop. Of course, it's easier when done...