If a terminally ill patient (example: on Hospice) is a full code at that point but the family cannot present you with a physical original and fully completed DNR, the EMT MUST begin resuscitation measures until the family can present the original DNR. You will have to work the code until you reach the hospital and the family can sign a new DNR at the hospital.
The ethics behind this are tricky but legally, you MUST work the code until the DNR is presented. However, to be kind, you don't have to go overboard with your interventions. You don't have to intubate the individual, you can do with a simple BVM and bag the patient while performing chest compressions.