The https://www.gemboxsoftware.com/test/pki/ is a simple Public Key Infrastructure (PKI) that contains the following hierarchy of certificates and CRLs:

• Root Certificate Authority certificate GemBoxCA.crt
  • Intermediate Certificate Authority certificate GemBoxRSA.crt
    • Certificate GemBoxRSA1024.crt
    • Certificate GemBoxRSA2048.crt (revoked)
    • Certificate GemBoxRSA3072.crt
    • Certificate GemBoxRSA4096.crt
    • CRL GemBoxRSA.crl that revokes the certificate with the serial number 02 (GemBoxRSA2048.crt)
  • Intermediate Certificate Authority certificate GemBoxECDsa.crt
    • Certificate GemBoxECDsa192.crt
    • Certificate GemBoxECDsa224.crt (revoked)
    • Certificate GemBoxECDsa239.crt
    • Certificate GemBoxECDsa256.crt
    • Certificate GemBoxECDsa384.crt
    • Certificate GemBoxECDsa521.crt
    • CRL GemBoxECDsa.crl that revokes the certificate with the serial number 02 (GemBoxECDsa224.crt)
  • Intermediate Certificate Authority certificate GemBoxDSA.crt
    • Certificate GemBoxDSA1024.crt
    • Certificate GemBoxDSA2048.crt (revoked)
    • Certificate GemBoxDSA3072.crt
    • CRL GemBoxDSA.crl that revokes the certificate with the serial number 02 (GemBoxDSA2048.crt)
  • CRL GemBoxCA.crl that doesn't revoke any certificate
  • OCSP responder certificate GemBoxOCSP.crt (currently inactive)
  • TSA certificate GemBoxTSA.crt (currently inactive)

To get a valid signature in your Adobe Acrobat Reader, you will have to add GemBoxCA.crt certificate to the list of Trusted Certificates using the following steps:

• Download GemBoxCA.crt certificate.
• Open Adobe Acrobat Reader, select Edit > Preferences > Signatures. In Identities & Trusted Certificates click More.
• Select Trusted Certificates and click Import. Browse to the downloaded GemBoxCA.crt certificate and click Import.
• Select the imported GemBoxCA <info@gemboxsofware.com> certificate, click Edit Trust and add a check-mark next to Use this certificate as a trusted root.
• You can now delete the downloaded GemBoxCA.crt certificate. You can always later remove GemBoxCA <info@gemboxsofware.com> from Trusted Certificates by selecting it from the list of Trusted Certificates and clicking Remove.

Otherwise, to get a valid signature in any Adobe Acrobat Reader, your digital ID would have to be an AATL-enabled signing credential.