As it is seen here, no currency variable is available : http://docs.whmcs.com/PDF_Invoice_Customisation
So what you need to do is simply make a database call within viewinvoice.tpl and get the currency variable.
Other and simpler way is to use whmcs built-in functions described here(Formatting Currency seciton): http://docs.whmcs.com/Useful_Functions_for_Devs