I think this positive result happened for the following two reasons: First, in forgiving others, people begin to see the inherent worth of those who offended. As this occurs, the forgiver begins to see that the self also has inherent worth. This tends to raise the self-esteem of the forgiver. Second, as people forgive, they begin to develop compassion for the offending person which tends to reduce anger in the forgivers. This reduced anger can lead to a reduction in anger, anxiety, and depression, all of which are associated with Post Traumatic Stress.
Reed, G. & Enright, R.D. (2006). The effects of forgiveness therapy on depression, anxiety, and post-traumatic stress for women after spousal emotional abuse. Journal of Consulting and Clinical Psychology, 74, 920-929. You can read the full study here.