Another option along the top of the screen is Add Shape.
After clicking on this option, choose with page you wish to add the Shape to. Once added, you can change the Shape, adjust the Opacity, choose if you want the Shape filled with a Color, select whether or not you want a Border around the shape, as well as the color and thickness of that border, and whether you want a Shadow applied to the shape. You can also move the shape backwards or forwards in relation to the other elements on the page.
In addition, you can click and drag on the end points to rotate the line or change the length. To remove a line, simply click on the Trash Can icon in the lower corner.