Digital signatures available for signing in ABBYY FineReader PDF 15 are located in the Personal certificate store of a user account. All certificates that contain a Private Key are shown in the application. Make sure you have a Trusted Root Certificate for the signature you want to use.

To see the list of the certificates available in the MS certificate Personal Storage:

1. Press Windows+R, type certmgr.msc and click OK;
2. Open Personal > Certificates;
3. Find the certificate you use for signing in FineReader PDF15;
4. Right-click on it and select Open and make sure that the certificate is valid;
5. Open the Certification Path tab and make sure that all certificates in the tree including the Trusted Root Certificate are valid.

After the document is signed with a valid signature, PDF-viewer may display it as invalid on a computer that does not have a Trusted Root Certificate which was used to sign the document.