Currently documents can be sent to be signed with strong authentication in Finland, Sweden, Norway and Denmark. In order to sign a document with strong authentication, the recipient must have one of the following:
• Finnish personal identity code and Finnish bank IDs or mobile ID
• Swedish personal ID and BankID
• Danish personal ID and NemID
• Norwegian BankID
If the signer owns none of the above, you can ask her/him to sign it without strong authentication.