Adobe Reader is a PDF file reader, with Form and Digital Signatures support.
How to import your Certificate into Adobe Reader
Open menu 'Document' 'Manage Trusted Identities'. Change 'Display' from 'Contacts' to 'Certificates'. Open the 'Add Contacts' dialog. With 'Browse' choose the cacert-root.crt certificate file. Click on 'Import'. Now the Cacert Signing Authority shows in the dialog. Select it and edit the Trust settings.
How to sign a form with Adobe Reader
How to make a PDF with Adobe Acobat
see Howto Adobe Acobat Dokumente mit CAcert Zertifikaten signieren (german, at the moment)