How can I overwrite existing file and keep same filename when uploading new

I have content type 'Document' and added the 'Upload document' field with type 'File' I want to replace file and update the node. I'm using below code

function custom_file_upload_form_alter(&$form, $form_state, $form_id) {
  if($form_id == "test_node_form") {
    $form['#submit'][] = 'custom_fileupload_submit';
  }
}

function custom_fileupload_submit(&$form, &$form_state) {
 $file = file_save_upload('field_document', array(
    'file_validate_extensions' => array('pdf'), // Validate extensions.
    'public://', FILE_EXISTS_REPLACE
  ));
  if ($file) {
    // Move the file, into the Drupal file system
    if ($file = file_move($file, 'public://', FILE_EXISTS_REPLACE)) {
      // Save the file for use in the submit handler.
      $form_state['storage']['file'] = $file;
    }
    else {
      form_set_error('file', t('Failed to write the uploaded file the site\'s file folder.'));
    }
  }
  else {
    form_set_error('file', t('No file was uploaded.'));
  }
}

Here file_save_upload() not working. I have debug the $file variable but it's print nothing.

You can just use the Upload File Replace module to solve this use case instead of custom code:

This is a small utility module that automatically stops Drupal from renaming new files upload via filefield CCK. When 2 files with the same name exist, the older files will be renamed.