Do adopted children have a higher rate of ptsd?

Possibly. Why certain people develop ptsd and others don't isn't clear. There is agreement that children subjected to traumatic life situations and experiences are more likely to wind up being put up for adoption. That may fuel the "common wisdom" that adopted children have a higher rate of ptsd than other children. If true it's most likely that trauma prior to adoption is to blame. Comorbidity exists also.
Trauma. The question is not whether or not the child has been adopted. The question is if the adopting child has been true able life threatening traumatic experience either real or perceived, with no hope of escape. It doesn't matter what country, state, region, city, or town they come from. All it matters is whether or not they have faced life-threatening trauma. That is what PTSD is defined as.
Depends . Children adopted after birth may have a higher risk for ptsd than those adopted at birth & subjected to no traumatic events in their upbringing. The older the adoptee, the more likely ptsd is to occur. A study from romania about the outcomes of adoptees in the netherlands indicated a high likelihood related to age at adoption. Many adoptees look for their biological parents when they're teens.