after applying the signature, do you get a prompt for saving the signed copy of the file? PDF Architect doesn’t sign the actual file directly, but creates a signed copy of the file. Did you get any error messages while creating the certificate?
Do you get the message about the invalid signature inside PDF Architect, or does PDF Architect say it was successful but if you load it in a different viewer, it says invalid? Please explain as detailed as possible, currently I am not able to reproduce the issue. What type of certificate did you use?