Thank you for reaching out about this. Digital signatures are used to authenticate the document and to ensure that it has not been further modified. As such, the usage of self signed certificates to certify the PDF is discouraged, as this does not guarantee the authenticity of the document. Please see the guide here for more information on digital signatures and obtaining a certificate from a certificate authority (CA).
Additionally, the typical workflow for digital signatures is to forward the document to a server to perform the certification. While this process can technically be done on the client side, providing the browser with the certificate to sign the document may introduce security implications.
Please see the guide here for some information about adding multiple signatures: Multiple Digital Signature on One Document | Apryse
Please also refer to this post for more info: What are the different types of digital signatures that can be added to a PDF using PDFNet?