The validity of document certification is UNKNOWN.
The author could not be verified.
If a .pdf document has been digitally signed with a third-party tool and the above message appears, then the following must be done.
The reason for this will be that Adobe cannot access or cannot access the Windows certificate store. The following must be set or activated for this.
After activation, that the Adobe Reader should check the signature and have access to the system's certificate store, it looks like this. The digital signature is now valid.
The same applies when using the Ascomp PDF Secure tool. After signing and opening the document, the signature may be invalid if the settings in Adobe Reader have not been adjusted.