Scope variable not getting data object after http service returns

You had mistaken several thing in your code.

1. Do return only promise returned by $http.get method from service. Because as you are trying to return a data from .success & .error callback of $http method. It will not return data back.
2. Do use .then function over service method call, on success which will call first function with data returned by $http.
3. You were expecting to get console.log($scope.listOfFilesOnCTP); to print data returned by service. But this will not return it. Async call are not work in this way. They will return out a data in special manner like promise resolution OR callback manner.
4. No need to use $scope.$apply here, as digest cycle has been already taken care by $http service.
5. Try to minimize the use of ng-init, you can do call that method on controller initialization itself.

Sevice

app.service('RdaService', ['$http', function($http) { 
  var self = this;
  self.getListOfFilesonCTP = function(path) {     
      return $http ({
      method: "GET",
        url: "../api/CTP/getFilesonCTP/"+ path,
        headers: { 'Content-Type': 'application/json' }
    });
  };
}]);

Then do use that promise while retrieving data inside controller.

app.controller('RdaController', ['$scope', 'RdaService', function($scope, RdaService) {
    $scope.listOfFilesOnCTP = "";

    $scope.GetListOfFilesonCTP = function(path) {
        $scope.listOfFilesOnCTP = RdaService.getListOfFilesonCTP(encodeURIComponent(path)).then(function() {
            console.log($scope.listOfFilesOnCTP);
        });
    };
}]);