Most people running downhill tend to slap the ground by long far strides and landing on heels with force, which is really painful. Even while running downhill they tend to lean back towards ground which put more strain on your back. Keeping a good posture while running downhill is really important. To keep a good posture ,you need to look forward, keep your head straight .Rather looking down to ground can strain your neck muscles. Fighting against the gravity can lead to early fatigue and decreased performance. So let our body move with the ground slope. If you want to decrease the pace ,try reducing your stride length. And never try to lock your knee joint while coming down, keep it a bit soft so as not to end up with any knee injury.