/**
 * Implements hook_field_access
 */
function MODULE_field_access($op, $field, $entity_type, $entity, $account){
  // Check view access to invoice field
  if ($field['field_name'] == 'field_pdf_invoice' && $op == 'view') {
    // Allow access if current user is viewing own account field
    if($entity_type == 'user' && $account->uid == $entity->uid){
      return TRUE;
    }elseif(user_access('administer users'){
      // Allow access to admins
      return TRUE;
    }
    // Default deny view.
    return FALSE;
  }
}