First, for your knowledge, the signing application must be executed from the customer side, not for server side, because from server side the application will never get access from the personal user token. In this scenario the best option is using OpenKM windows client signature, in the latest version the application is able to sign documents from OpenKM ( document might be signed from computer customer side, but the process is transparent for him, that is mandatory because you must have the token, the document and the signing application in the same context, here remote sign is not possible ). I'm not 100% sure if community version supports the signature of existing documents into OpenKM, this is a feature we added some months ago in the professional version and I'm not sure if community also has it.
I'm talking about it
https://www.youtube.com/watch?v=CNIskRk5VCY
Other scenarios should need customization and thirdparty software integration. For example exist the possibility of integrate this software
https://blobfish.pe/productos/blobfish-signer/