Issue #3276500: pdftk binary check fails

fillpdf.module

@@ -2076,7 +2076,7 @@ function fillpdf_pdftk_check($pdftk_path = 'pdftk') {
     return FALSE;
   }
   // Check if version output includes "pdftk".
-  elseif (!strpos($output, 'pdftk')) {
+  elseif (strpos($output, 'pdftk') === FALSE) {
     return FALSE;
   }
   return TRUE;