Digital Signartures in pdf documents in version 7.6

Problem in digital signatures on .PDF file.
My system configuration Ubuntu 20.04.1
LibreOffice Version 7.6
Problem Description:
Purchased digital signatures from eMudhra.com (India)
They delivered USB Token containing digital signatures.
Despite my several efforts, I am unable to digitally sign the .PDF file.
USB Token is attached to my desktop through USB Port
On selecting File>Digital Signatures>Digital Signatures and running Certificate search, no DSC file is visible.

Have you looked at theHelp for Applying Digital Signatures?

I have purchased Digital signatures from eMudhra (India). To recognize the signatory, eMudhra issues one USB Token, which is attached to any USB post of the desktop.
Whenever I try to digitally sign any .pdf document ( opened in LibreOffice Draw ) it searches location of Digital Signatures.
At this stage, attached USB Token is not recognized by LibreOffice Draw.
Accordingly I request you to please guide me how to configure the attached USB with LibrOffice Draw.

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