Additional custom PDF files with invoice

Hi, we are exploring Zoho Invoice. One of our requirement is to send few more PDF documents (certificate, etc) along with each invoice. Those documents will also carry a reference to the item and invoice meta information (like description, invoice number etc).

Can any one tried this before, if so provide some pointers?

PS: I can create a custom template but that may not help here as I need to send 4 documents in total (including invoice) for each sale.