The Adobe Reader software does not by default recognize signatures made by A1 Digital Certificates issued by ICP-Brasil.
As a result, Adobe Reader warns the user that the signature is unknown.
To resolve this issue, simply import the Brazilian Root v2 into Adobe Reader.
WARNING: The document is properly signed, ensuring its integrity. The message displayed by Adobe Reader is only due to the lack of the Brazilian Root as a default in the software.
Open Acrobat Reader and click on "Edit – Preferences"
Click on "Signatures" and in the "Trusted Certificates and Identity" section click the "More" button
In the left menu, click "Trusted Certificates" and select "Browse"
Select the .p7c file containing the full ICP-Brasil Root chain and click "Open"
Confirm that the certificates appear in the list and click "Import" for Adobe to incorporate the necessary certificates
Back on the settings screen, in the right list, click on the imported Brazilian Root Certificate and click "Edit Trust"
In "Edit Certificate Trust", check the "Trust" tab option "Use this certificate as a trusted root" and click OK
Click OK and close all windows. Restart Adobe Acrobat. To test, simply open a signed PDF document. Adobe Reader will verify the certificate traceability chain and indicate in the blue bar at the top of the document that the digital signature is valid
Done! Your Adobe Acrobat will be configured and ready to correctly validate signatures based on certificates issued by entities subordinate to the Brazilian Root Certification Authority