Unfortunately, I don't see that there's a keyboard shortcut for the pencil tool, as there is for other commenting tools, so I don't think this is possible.
George is on the money - you'll not be able to use the pencil tool while in full screen.
Something to try:
Use F8 and F9 to hide what is at the top of Acrobat's window.
Have the Pencil tool selected for use first.
You'll be able to use the Pencil tool.
The PDF, with Acrobat's window maximized, will give the appearance of "full screen".
Thanks guys, I realize it's not really meant to be presentation software, but CtDave's suggestion will work fine. Thanks!