When trying to display the PDF invoice received from Isabel, it can happen that you see the warning ‘At least one signature has problems’. 

This can happen when viewing documents through Zoomit, but also for already downloaded documents.

The solution is to add the certificate for the “Isabel Invoice Signer” to the trusted certificates:

Click on Signature panel

In Signatures Select the Signed by Isabel Invoice Signer Signature

Click on Validate Signature

Click on Signature Properties

In the Signature Properties, Select Show Signer's certificate...

Go to the Trust Tab and in the left pane, Select the Isabel Invoice Signer certificate

Click on Add To Trusted Certificates

Click on OK

And in the next window, Check Use this certificate as a trusted root and Click on OK

Close the PDF document and open it again. The Signatures are now signed and valid