title: Android - secure app data on external storage
body: <p>I need to secure my apps data securely on external data. Is there a way to hide data from getting accessed via other apps or by USB?
  There is lot of data so don't want to use internal storage in private mode.
  How can I set permissions on a file like chmod 700 ?</p>
  
  <p>App data is PDF file which need to be opened in PDF readers. So need to take care of that also</p>