Based on the source you can use the 'dataurlnewwindow' parameter for output():
doc.output('dataurlnewwindow');
Source in github: https://github.com/MrRio/jsPDF/blob/master/jspdf.js#L914
All possible cases:
doc.output('save', 'filename.pdf'); //Try to save PDF as a file (not works on ie before 10, and some mobile devices)
doc.output('datauristring'); //returns the data uri string
doc.output('datauri'); //opens the data uri in current window
doc.output('dataurlnewwindow'); //opens the data uri in new window