diff --git a/htdocs/core/lib/pdf.lib.php b/htdocs/core/lib/pdf.lib.php
index 735e6892fcd..b7ef5925748 100644
--- a/htdocs/core/lib/pdf.lib.php
+++ b/htdocs/core/lib/pdf.lib.php
@@ -35,6 +35,8 @@
  *	\ingroup    core
  */
 
+include_once DOL_DOCUMENT_ROOT.'/core/lib/signature.lib.php';
+
 
 /**
  *  Return array head with list of tabs to view object informations.