This may be simple but I just wanted to ask. I want to be able to see in the system information that a Purchase Order has been printed. Do I need to create a custom field that is change from F to T when the PO is printed? Can this be done via workflows?

There's no event that is fired when you press the print icon. I just tested this as well to double check. No UE fires when the button is pressed. You'd need to add a custom button for printing and then after it prints set the custom field to true.

@Tim Chapman how do you print the POs? I've scripted a print button for clients, and on click of that button, it toggles a checkbox. In that case, we hide the native print buttons so the only way to print is with the custom button. That way we can track what has been printed and what hasn't. I've also utilized printnode in the past to send things directly to the printer. So a transaction can be saved, sent to the printer, and marked as printed all at once.