Are there any difference in the functionality of a free and paid plan?

Both free and paid plans have access to the same set of functionality but the free plan is more limited in terms of storage and team size. Furthermore will inactive data rooms under the free plan have the assigned sub domain released. The data room will still be manageble from the management interface and a new sub domain can be added again at a later stage.

How do I redeem a voucher?

If you have a voucher to redeem please log in via https://my.pitchroom.io and then go to Account > Billing and fill in the voucher in the voucher form