[PI-Ariba] Error while attach a PDF file into cXML PurchaseOrder outbound interface to Ariba

Dear SAP PI/PO experts

I was trying to attach a pdf file into purchase order interface outbound to Ariba in this mapping

MM_Custom_ORDERS_ORDERS05_2_CDF_cXML_XSD

Here is the error that I got:

Mapping "http://ariba.com/xi/ASN/PurchaseOrder/IFM_ORDERS_ORDERS05_2_cXML" failed to execute: MappingException: Mapping failed in runtimeRuntime Exception when executing application mapping program com/ariba/asc/sap/netweaver/xiadapter/javamappings/OutboundAttachmentHandler; Details: java.lang.NullPointerException; while trying to invoke the method org.jdom.Attribute.getValue() of a null object returned from org.jdom.Element.getAttribute(java.lang.String), ApplicationRuntimeException: Runtime Exception when executing application mapping program com/ariba/asc/sap/netweaver/xiadapter/javamappings/OutboundAttachmentHandler; Details: java.lang.NullPointerException; while trying to invoke the method org.jdom.Attribute.getValue() of a null object returned from org.jdom.Element.getAttribute(java.lang.String), NullPointerException: while trying to invoke the method org.jdom.Attribute.getValue() of a null object returned from org.jdom.Element.getAttribute(java.lang.String)

Here is my cXML for attachment part:

Here is my OM

Anyone has any idea about this?

My guess is our PI can't connect to outside which mean that once it retrieved the URL outboundHandler.class cannot download the file content.

Do we have to maintain files in SAP Doc Server?

Thank you in advance everyone.