How to sign a PDF using a certificate (.pfx file)

We need to sign PDF documents with a certificate (.pfx file). Seems that ACROBAT SIGN App (iOS) do that but we can't find this option in the App.

We've activated the trial licence (so we're on 30 days trial period). Also we have a CREATIVE CLOUD Enterprise subscription.

After 1 hour in contact with your support I can't receive assistance because we're a trial user.

Thank you in advanced for your help.