Some Adobe users in my company see when they opened signed documents - "The validity of the document certification is UNKNOWN. The author could not be verified"

As implied by ​@Amal Jaiswal ‘s answer there apparently is a difference in determining trust on the different computers.

The cause might be that more certificates are explicitly trusted on some computers. In that case adding trust anchors as proposed by Amal on the other computers would be appropriate.

Another cause might be that on some computers Acrobat is configured to trust OS (operation system) managed certificates and on others not. In that case your IT department should decide whether trust in the OS managed is appropriate or not and configure the Acrobat installations equally.

The cause might also be, though, that some intermediate certificates to build chains to the trusted roots are missing. In that case a better fix would be to install the missing intermediate certificates everywhere.

To determine which of these possible causes is the case at your place, you need to compare the details of why the signer certificate is trusted on some computers and not on others.

Hi there 

This error occurs because the signer's digital certificate isn't in your Adobe Acrobat list of trusted identities. To fix it, you need to manually trust the certificate and validate the signature.

Try the steps below and see if that works:

1. Right-click (or Ctrl+click on Mac) the signature with the unknown validity and select Show Signature Properties.

2. In the dialog box that appears, click the Show Signer's Certificate button.

3. Go to the Trust tab, then click Add to Trusted Certificates.

4. Click OK if prompted, and ensure you check the box that says "Use this certificate as a trusted root".

5. Click OK on the trust windows, then click Validate Signature.

For more information, please check the help pages listed below:

https://adobe.ly/4aaFYNi

 https://adobe.ly/4vl6USP 

~Amal