FreePDF: The The Manage certificates function is not included in FreePDF.
You can add trusted certificates to a list so that FlexiPDF knows about them in future. You should only do so if you are sure of the origin of a certificate.
To mark a certificate as "trusted", proceed as follows:
▪Click on the certificate in the Signatures toolbar and then on Add.
If the sidebar is not shown, open it with the ribbon command View | group Window | Sidebars  | Digital signatures. |
▪Alternatively, use the ribbon command Document | group Digital signature | Manage certificates 
to open the dialog box "Manage Trusted Certificates", and click on Add there.
Lists of trusted certificates
FlexiPDF uses publicly available lists of trusted certificates to identify when a signed PDF might be a cause for concern. To ensure that you have the latest information about which certificates can be trusted, you should update your copy of the lists occasionally.
To update the public list of trusted certificates, proceed as follows:
| 1. | Use the ribbon command Document | group Digital signature | Manage certificates to open the dialog box "Manage Trusted Certificates". |
| 2. | Click on the Update now button. |