Issue #1866174 by Pancho: Validate pdftk binary

src/Component/Utility/FillPdf.php

@@ -30,13 +30,11 @@ class FillPdf {
     if (empty($pdftk_path)) {
       $pdftk_path = 'pdftk';
     }
-    $process = new Process($pdftk_path);
+    $process = new Process($pdftk_path . ' --version');
     $process->run();
 
-    if (in_array($process->getExitCode(), [126, 127], TRUE)) {
-      return FALSE;
-    }
-    return TRUE;
+    // Check there's no error code and the --version output includes "pdftk".
+    return !in_array($process->getExitCode(), [126, 127], TRUE) && strpos($process->getOutput(), 'pdftk') !== FALSE;
   }
 
   /**