How do I get a digital certificate? You can buy a certificate from a Certificate Authority (CA). Specify that you want to digitally sign documents. This process takes some time because the CA will go through a validation procedure. If you intend to sign PDF it is a good idea to select a supplier from the Adobe Approved Trust List - AATL root certificates are automatically trusted by ADOBE applications. Other vendors root certificates will have to be explicitly installed in ADOBE. An ADOBE installation maintains its own list of trusted CA - it is not sufficient for a certificate to be in the TRUSTED ROOT STORE.