Attach Invoice document in FBL5N- Attachment list from Archive link

Hi

Kindly help me with the following requirement.

I need to develop a code to attach the invoice document in FBL5N Attachment list. The invoice document will be present in the spool. I have the spool ID.

I want to retrieve the PDF using the spool ID and populate it to the attachment list of FBL5N customer line item.

Can you please help me to achieve this?