Workflows do not currently support a print feature. However, the summary activity does support a print button. To print the summary activity, first drag a summary activity into your flow from the palette and then setup summary fields to indicate which form fields to display in the printable summary activity.
Designers can click the 
in the toolbar to see a preview of a customized pdf in the flow designer.
It is also possible to add your own print button to any workflow step by adding a message control with the following html. Note that a custom print button will print the current activity step and not the entire workflow. Also the current activity will print even if the activity's print property is unchecked.
<a title="Print this form" onclick="_frevvo.api.printFlowStep(this);" href="#" class="f-tb-print"><span id="f-form-tb-print-span">Print Form</span></a>
See printing forms for more details on how to add a print message control.