PDF Conversion in sap for invoice

Hi ,

Help required in PDF conversion .

Im using OTFSPOOLREQUESTPDF to create the invoice form VF31 to PDF individually and attach to mail .

My requirement is for a range of Invoices I need in a single PDF of all the Invoices after VF31 BDC .

Should I use some other Function module ?

Kindly help .

Regards

Severina