copy downloaded png to Documents folder?

How would I copy a png from a server to the documents folder. would I do something like this

 download = new WWW (url);
     
 yield download;
     
 pngbytes = download.bytes;
 System.IO.File.WriteAllBytes(Application.dataPath + "/Documents/mypng.png", pngbytes);

If the png is in an assetBundle how would I do it?

 download = new WWW (url);
 yield download;
 currentAssetBundle = download.assetBundle;
 mypng=currentAssetBundle.Load("mypng");
 ?