The next patch shows how to do this for Invoices:

diff --git a/modules/Invoice/InvoicePDFController.php b/modules/Invoice/InvoicePDFController.php
index f1a0605..25236c0 100644
--- a/modules/Invoice/InvoicePDFController.php
+++ b/modules/Invoice/InvoicePDFController.php
@@ -58,7 +58,7 @@ class Vtiger_InvoicePDFController extends Vtiger_InventoryPDFController{
 	}
 
 	function getWatermarkContent() {
-		return $this->focusColumnValue('invoicestatus');
+		return ''; //$this->focusColumnValue('invoicestatus');
 	}
 }
 ?>