This issue is resolved in GroupDocs.Total API version 24.6.
I am able to sign the document now using Groupdocs Signature but the problem is I am not able to place the signature it is by default getting placed in top left. pages_Files (1).pdf - Google Drive
Please share sample code with us. We need to see how you are setting signature position in the code.
To check the PFX certificate for validation you do not need GroupDocs.Signature or Total GroupDocs.Total APIs.
Youu can check the .NET X509Certificate2 class with its properties and Verify() method.