Attaching a Document if a checked box is checked

[lynnw43865805]

I have a lengthy document for accounts to sign.  There is one section that if they check the box that is YES, they are supposed to email a specific form they have to verify the YES.  

Is there a way, if they check the yes box, that after they sign, that it requires them to attach the specific document from their files?  It is their proof of Tax Exemption, and because they have several pages to read, they forget to send it after they have signed.  It would be real slick if there were a way that I could make it so that it made them attach their document, before it will send to the next signer.