B777-200 pilot reported the door actuator for the escape slide was activated when ramp crew opened the door after a ferry flight. The Captain prevented slide deployment and maintenance was called.

Narrative

After shut down on the ramp I exited my seat and went to door 1L. I wanted for the ramp crew to pull up the air stairs to the airplane. It took them a while to get the stairs to the plane. Once the stairs were in place; I continued to wait for the ramp crew to come open the door from the outside. I heard the Captain ask if the stairs were there. I replied the stairs were in place. After another moment or so I stepped away from the door for a moment to gather my things. While I was gathering my belongings I heard the door being opened and a hiss that I after found out was the door actuator being activated. The Captain was holding the door closed trying to prevent the slide from deploying. After a few more moments a maintenance person was able to come through the partially open door and unhook the girt bar from the door and they let the door open the rest of the way.I suggest a section be added to the manual specifically for maintenance ferry flights as it is not something that we do regularly. We are qualified to do it but only talk about the procedures during recurrent.