Well it depends on the laws of each country. In my country (Greece) a digitally signed document is acceptable everywhere at least in the public sector. Actually it's illegal for a public servant to deny a digitally signed document!
No professional service is really needed to sign a document; it all depends on the acceptance of the certificate you use for signing by your government/laws. I.e you may need to buy a certificate from a trusted organization or you may need to generate a certificate from a public sector organization of your country.