At any time you can add a drawing border to the design.
Using the Add Component browser (<F9>), select the library from the drop down list.
For reference, this is a schematic only component library and will not have any PCB symbols associated with them.
Scroll down the list and select the A4 drawing border. This has been previously created to fit an A4 page size.
Note: there is no PCB footprint for this component, it is a Schematic only.
Like other components, select A4 from the list or from the preview. Drag it into the design and position it on top of the existing components. You may need to zoom out a bit to see it all to place it somewhere near. Use the <Z> key to zoom out or <A> to view all of the design. For speed you can also roll the mouse wheel.
As with other components, this can be moved and edited later if you need to, extra detail can be added to it. Click once to release it.
How can I place components?