sign in required to sign document?

HI

I have sent out a document to number of people to sign, they have come back saying that they are being asked to sign up to Adobe (and pay  a fee) before it will let them sign.

How do I change this?  My account is a business account and I have sent out a load of documents to sign without any problems so not sure why this one is behaving differently?

Any suggestions welcomed

thanks

Nicola