Document template - Customize print file name with mergefields

When I print a fulfillment pickslip it saves under the same name each time:

FulfilmentPickSlip SO-XXXX for (client Name).pdf

I'd like to have control over that pdf name, preferably using merge fields, so it at least saves as:

FulfilmentPickSlip SO-XXX/(Fulfilment Number) for (Client).pdf?

Right now I have some orders with perhaps 10 fulfillment pickslips and they're all called the same thing - it's quite frustrating.