Hi Rajeev ,
SE38 is a transaction for ABAP development and is not normally assigned to end users. Sometimes SA38 is enabled in special cases. Both transactions are solutions to convert a single spool order.
This document created by Brendan O'Callaghan can help you Creating or Converting Large PDF Files
For mass creation definitely you need abap help to create a program or LSMW.
If you do not need a pdf file for each purchase order, maybe you coulld create a single order spool for all of them and then generate a single pdf file, but I think you need a file for each purchase order.
Best regards,
Alejandro López.
Message was edited by: Alejandro Lopez R