Hello Prasad,
if I understand correctly and transaction SOST shows a correct PDF attachment, but an unreadable PDF attachment arrives to Outlook, you can check a SOST tracing.
SAP KBA #1947273 - "How to save the MIME header for problem analysis?" shows how can you save the MIME data which is sent from SOST.
After you have created the MIME trace, you can it and compare with the "result" in Outlook to see whether the same data has been sent and received.
Best regards,
Laszlo