Download.php: Skip 'pdftk' if there is only one PDF file to concatenate.

extension/qfq/qfq/report/Download.php

@@ -76,6 +76,15 @@ class Download {
      */
     private function concatPdfFiles(array $files) {
 
+        switch (count($files)) {
+            case 0:
+                return '';
+            case 1:
+                return $files[0];
+            default:
+                break;
+        }
+
         $concatFile = tempnam(sys_get_temp_dir(), DOWNLOAD_FILE_PREFIX);
         if (false === $concatFile) {
             throw new DownloadException('Error creating output file.', ERROR_DOWNLOAD_CREATE_NEW_FILE);