B757-200 First Officer reported the escape slide pack fell out while trying to close the doors; citing pads on aircraft floor as a contributing factor. Maintenance was called.

Narrative

Upon arrival to the aircraft; moderate rainy conditions were present for the last few hours. Once pre flight and cabin inspection complete; we decided to close the doors to stop more rain from coming in at the door entrance. Pads were placed on the floor by ground crew to soak up some of the water. I; the first officer; closed the doors and reported to the captain that both doors were closed and disarmed. As we went through our normal cockpit preflights and briefs; the DG (Dangerous Goods) agent opened the door and half closed it behind her. She gave us the paperwork; and waited in the waiting area. There was no DG inspect. Once we had a final wt and bal; I went to the waiting area to give the signed DG paperwork and release the DG agent from the aircraft. The captain instructed me to pay particular attention to the water in the door area; to make sure it was not excessive. The DG agent had trouble opening the door to leave; so I assisted to open the door. As she walked and went down the stairs; I closed the door partially with the handle at the 1 o'clock position. The door closes fully when at the 3 o'clock position. As the door was closing I notice that the floor pads were lodged underneath the door. I stop closing the door to remove the pads to ensure they were not in the way of the door seal. I pulled on the pads but they appeared to be stuck. I then opened the door more to try and get the pads out. Upon opening the door; the slide pack fell out and rested on the stairs. The door was never armed as I never closed it fully (door handle in the 3 o'clock position) The slide did not extend or deploy. I also noticed that the girt bar was extended; and that there were two pads; one pad lodged underneath the girt bar and floor; as well as one on top of the girt bar. We called maintenance to the aircraft. Maintenance removed the slide pack and stowed it in the cargo bay netting area. They MEL'd the left door inop; and wrote the Aircraft Maintenance Logbook entry with the verbiage; left slide fell out due to possible FOD. We waited a few minutes for our flight ready paperwork; and then proceeded to continue the flight.Cause: The lodging of the pads on the bottom rail of where the door closes got pinched between the girt bar and the rail. This led to the girt bar being partially stuck which I believe caused the door slide to dislodge upon closing and opening the left door.Suggestions: In the future; any objects or FOD in the doorway should be paid more attention to. In this case; the pads were necessary to remove as much rain water as possible. Rain was heaviest at the door entrance; so the pads were necessary there. However; I would remove those pads before closing the door just to be sure they don't get caught near the rail way of the door where the girt bar sits.