You can add either an electronic signature or a digital signature.

Electronic signatures are cheaper to implement and can only be verified on the 'source' system

Digital signatures require secure certification and have the highest form of security - these are independent of the 'source system'.

Hand written signatures for SOPs may be substituted with electronic signatures - however you must consider whether the integrity, authencity and context of the electronic signature will be maintained through the documents entire retention period ( including archiving ).

Therefore it is recommended digital signatures are embedded into the documents and more appropriate of long term retention.