Adobe Acrobat shows this error when S/MIME certificate has been used to digitally sign a PDF document. Adobe and Sectigo are offering a new product, specifically targeted for document signing. Citation from Sectigo Support:
Greetings from Sectigo Support!
Yes, the Personal Authentication Certificate which you've purchased will give a trusted identity error when you sign Adobe PDF files. Instead, it is recommended to purchase a new certificate specifically designed for document signing which comes with a new trusted chain " Sectigo RSA Document Signing CA” added into Adobe Approved Trust List (AATL).
Please do refer the following link for more details about Sectigo document signing certificate.
Please let us know if you need any further support.
Sectigo - Technical Support
Solution: kindly obtain a Sectigo Document Signing certificate (InterSSL is currently not offering this product).