Can't sign pdf documents with Okular in Debian

[Adriano Vilela Barbosa <adriano.vilela@gmail.com>]

I'd like to know if anybody can sign pdf documents using Okular in
Debian as explained in the manual:

https://docs.kde.org/stable5/en/okular/okular/signatures.html

I've never been able to do this, although I have a PKCS signing
certificate available at $HOME/.pki/nssdb. In Okular, the menu entry
"Digitally Sign" under the "Tools" menu is grayed out and the section
"PDF Digital Signature Certificates" of the PDF backend configuration
(https://docs.kde.org/stable5/en/okular/okular/config-pdf.html#config-pdf-digital-signatures)
doesn't exist on my system.

At the bottom of the page on the first link above there is a note that says:

"This feature is available for PDF documents only. You need to install
at least version 21.01 of Poppler to be able to add digital signatures
to a document."

I'm running Debian Testing and, until yesterday, the version of
libpoppler installed on my system was 20.09. Thus, I figured pdf
signing wasn't available on my system because I didn't have the
required version of libpoppler. Last night an update came along which
brought libpoppler from version 20.09 to version 22.02. However,
nothing changed after the update: I'm still unable to sign pdfs.

So, I wonder if anybody here is able to sign pdfs using Okular in
Debian and, if so, what's required to get that working.

Thanks a lot.