Generally, many Saipan have certificate restrictions. You go to the mobile phone house to register a number, click on the Saipan certificate and apply for the certificate. You will be prompted. Do it. After applying, you can enter here to sign online, that is, upload what you want to sign, and then download a signed one. You can also download the mobile phone signature assistant to your mobile phone and sign it later. Signature only supports files in sis and sisx formats.