String pathFile =
    Environment.getExternalStorageDirectory().toString() +
    FileUtils.PATH_SEPARATOR + "myFolder" +
    FileUtils.PATH_SEPARATOR + "myFile.pdf";
File pdfFile = new File( pathFile );
if ( pdfFile.exists() ) {
    Uri uri = Uri.fromFile( new File( pathFile ) );
    Intent intent = new Intent( Intent.ACTION_VIEW );
    intent.setDataAndType(uri, "application/pdf");
    intent.addFlags( Intent.FLAG_ACTIVITY_NEW_TASK );
    startActivity( intent );
}

<uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE" />
<uses-permission android:name="android.permission.READ_EXTERNAL_STORAGE" />