Issue #1392922: Explain why strlen() is used.

fillpdf.module

@@ -425,6 +425,8 @@ function fillpdf_merge_pdf($fid, $nids = NULL, $webform_arr = NULL, $sample = NU
   drupal_add_http_header('Expires', 0);
   drupal_add_http_header('Cache-Control', 'must-revalidate, post-check=0, pre-check=0');
   drupal_add_http_header('Content-type', 'application-download');
+  // This must be strlen(), not drupal_strlen() because the length in bytes,
+  // not in characters, is what is needed here.
   drupal_add_http_header('Content-Length', strlen($data));
   drupal_add_http_header('Content-disposition', 'attachment; filename="' . $output_name . '"');
   drupal_add_http_header('Content-Transfer-Encoding', 'binary');