The sender can Delete a signature request if he has initiated it by mistake or missed adding a signer or requested for a wrong document. Don't worry! Just open the Pending document, and tap on Cancel option. Once you have confirmed cancelling a signature request, all the signers receives a push notification as well as an email notifying him/her that the signature request has been cancelled.
PS: Request Signature feature is available on app version v4.2.0 and higher, under Business plan.