title: Protect files from being accessed outside the app itself
body: <p>I am developing an android application, where I download pdf files.
  So I create a file called <code>NameProject</code> where I put all downloaded pdfs. I can access this file via my application and also via other application like File Manager.
  The android file system is open so I need to protect downloaded files from being accessed outside the app itself.</p>
  
  <p>Can I do that? If yes, any ideas will be appreciated.</p>
  
  <p>Thanks</p>