Some digital presses give users the option of printing an invisible "void" pattern that shows up when copied. If your press doesn't have that, digital void pantograph technology can be purchased from a provider of print security devices. Another option is to purchase paper with the void pantograph pattern pre-printed in the background. Finally, if those options aren't effective, red fluorescing dry ink is available for many digital presses. It produces invisible images that only show up under UV light.