Hi Rajeev ,
SE38 is a transaction for ABAP development and is not normally assigned to end users. Sometimes SA38 is enabled in special cases. In both cases, are solutions for a single spool order.
This document create by Brendan O'Callaghan can help Creating or Converting Large PDF Files
For mass creation definitely need help abap program or LSMW.
If you do not need a pdf file for each purchase order, maybe you can 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.