Are you using Acrobat? or FormsCentral to make a form? FormsCentral currently doesn't support signature fields. You might also want to check out our sister service EchoSign (www.echosign.com) - they make signing documents really easy.
I have both Acrobat Pro and FormsCentral but I need signatures,
mandatory fields and the ability to return the form. Since our field
staff use iPads it seems I can't have all three things at once.
Checkout EchoSign. They support PDF files and I beleive they can meet all of your requirements.