Would like to inform you that using 'Certificates' tool, you may place your signatures over the document.
Also refer to these Kb articles :- https://helpx.adobe.com/acrobat/using/digital-ids.html
Thank you for your time & energy on helping resolve this issue. Thank you for the resources that proved to be helpful. After digital signature was created ( following all directions), I opened the document to be signed. Clicked on Certificates to find the options at the top of the page not enabled or ready to use. Under Certificates I see the option Digitally Sign, but it is inactive. Any thoughts on how I can activate these options so I can move forward with signing this document?
A bit of history: The document was not created by me. It is a contract created with the help of legal supervision and sent to me for signing. The author of the first signature ( I am the second) could not be verified. Could this be a reason I am having difficulties?
YatharthS, thank you for your help.
It looks like your Acrobat installation does not have trust for the signer of the first signature and because of that the signature status is unknown. Right-click on the first signature, select 'Signature Properties' and then 'Show Certificate'. The 'Certificate Viewer' dialog comes up. In the left pane there is the certificate chain. You can select the top (root) certificate, inspect it, and add it to Trusted Identities if you think that it is indeed trustworthy. Click on the Trust tab above the right pane to do that. Be careful to add tot he Trusted Identities the roots that you know can be trusted.
If the chain has only one certificate then it means that the first signer signed with so-called self-signed certificate which does not provide means to ascertain its authenticity. It is up to you to decide whether you trust this signer. If you do, add it to the Trusted Identities.
You do not completely trust the first signer you can add root certificate to the Trusted Identities, do your signing and then go to Preferences->Signatures->Digital IDs&Trusted Certificates->More... and remove certificate you just added to Trusted Identities so that it is not there anymore.
Your PDF may be certified without enabling permission to add digital signatures or signed with locking the document. In either case this prevents from adding more signatures to the document.