You can digitally sign files either when you add them to an archive or in an existing archive.
Enable Signing in Existing Archive
Central directory and individual files
If the Sign drop-down menu is set to Central directory, you cannot sign files already in an archive, and the Sign button on the Actions panel is disabled.
Note: If signing is toggled off when you sign files that are already in an archive, only files are signed. The archive's central directory is not signed in this case even if Central directory and individual files is selected in the Sign drop-down menu list.
To sign files that are already in the current archive
1. In the main window, select particular files to sign, or deselect all files (click in an empty area) if you want to sign all of them.
If no files are selected, all will be signed. Selecting zero files has the same effect as selecting all files.
2. To sign the files, click Sign on the Actions panel.
If some files you selected are already signed, PK Protect asks if you want to sign them again. If you do, PK Protect signs the files using the certificate currently selected as the default on the ZIP or OpenPGP pages of Security options.
If some files you selected are encrypted, PK Protect asks you for the passphrase, if necessary. PK Protect needs to temporarily decrypt the files in order to sign them. After they are signed, the files have the same encryption as before.