This file shows how to specify the PDF layers in the document that can be switched between visible and invisible states. Check Layers panel of Adobe Acrobat/Reader to see the layer named "Answers". 9×1 = 9 9×2 = 18 9×3 = 27 9×4 = 36 9×5 = 45 9×6 = 54 9×7 = 63 9×8 = 72 9×9 = 81