Looking for the realpath of a cached file

I write a module for a new field where I need to get access with imagemagick identify (not convert!) to a not saved file.
More precisely, I upload a pdf file in a widget. In a validation function I want to identify the size of a pdf.
The file before saving is like the following:

stdClass Object
(
    [uid] => 1
    [status] => 0
    [filename] => bla.pdf
    [uri] => /tmp/phpDS79Xg
    [filemime] => application/pdf
    [filesize] => 70322
    [source] => field_imgext_und_0
    [destination] => public://bla.pdf
)

Because I want to handle the file with shell_exec ("identify -format "%[pdf:HiResBoundingBox]" bla.pdf") I need the location of the cached file.
In my drupal conf at admin/config/media/file-system the temporary directory is sites/default/files/tmp. But I don't think that this has something to do with my issue because validation is before saving!

Because I want to exceed the image module I can strongly orientate the processes from this. The image module uses

getimagesize(drupal_realpath($image->source))

When I debug drupal_realpath, which is:

function drupal_realpath($uri) {
  // If this URI is a stream, pass it off to the appropriate stream wrapper.
  // Otherwise, attempt PHP's realpath. This allows use of drupal_realpath even
  // for unmanaged files outside of the stream wrapper interface.
  if ($wrapper = file_stream_wrapper_get_instance_by_uri($uri)) {
    return $wrapper->realpath();
  }
  // Check that the URI has a value. There is a bug in PHP 5.2 on *BSD systems
  // that makes realpath not return FALSE as expected when passing an empty
  // variable.
  // @todo Remove when Drupal drops support for PHP 5.2.
  elseif (!empty($uri)) {
    watchdog("realpath",  print_r($uri));
    return realpath($uri);
  }
  return FALSE;
}

(watchdog from me). The realpath results in 1. (Which perhaps means TRUE). So there is a realpath and no file_stream_wrapper???

Do I have to save the file before I can get the size of the pdf???

Any ideas are welcome!!!

Thx, maen