The libpoppler-utils package contains the tool pdfsig that does just this.

$ apt install libpoppler-utils
$ pdfsig file.pdf
Internal Error (0): couldn't find default Firefox Folder
Digital Signature Info of: file.pdf
Signature #1:
  - Signer Certificate Common Name: Donald Duck
  - Signer full Distinguished Name: C=SE,E=donald.duck@stackoverflow.com,O=Uppsala universitet,CN=Donald Duck
  - Signing Time: Oct 10 2021 15:12:20
  - Signing Hash Algorithm: SHA-256
  - Signature Type: adbe.pkcs7.detached
  - Signed Ranges: [0 - 132117], [137169 - 143786]
  - Total document signed
  - Signature Validation: Signature is Valid.
  - Certificate Validation: Certificate issuer is unknown.

The complaint about Firefox folder goes away if you simply create the empty folder ~/.mozilla/firefox.

• https://stackoverflow.com/questions/41301138/how-to-check-if-a-pdf-has-any-kind-of-digital-signature/67331180
• https://askubuntu.com/questions/226257/how-can-i-validate-a-pdfs-digital-signature-with-evince
• https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=926404
• https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=924050