why $cordovaFile don't create a folder?

Question

i'm trying to create a folder with cordovaFile ngCordova, but it always call the error callback, some advice?

$cordovaFile.createDir(cordova.file.applicationDirectory+"/www/pdf/","attach", true)
          .then(function (success) {
            console.log('folder created');
          }, function (error) {
            console.log('folder not created');
          });