Adding Digital Signature in PDF Report

Dear Sir,

My client currently generates PDF files from NAV/RTC Report. After that they attache a Digital Signature(3rd Party) into it & mail. The Digital Signature is located in a PEN DRIVE provided by a 3rd Party. Only 3rd Party Software can locate/fetch the signature file from the PEN DRIVE.

How can I attache the same file direct from the PEN DRIVE into the PDF Report without any user interaction ?
Kindly guide.

Thanks,