Yes, the statement of your booking you can print our from your 9flats account. Simply go to Trips-> My trips and select 'All'. Next to your booking you will see button 'Invoice', click there to download the invoice for the guest service fee.
For some companies, the statement you will find in your 9flats profile it's enough, but please remember that this is an invoice just for the guest service fee, not for total amount of your booking.
To receive an invoice for your whole stay, please ask your host to send you a receipt. You are entitled to ask for proof of the total amount of your booking.
Every host is oblige to send you an invoice for the amount he collected for your stay.