pdf add-on problem

Hi there

I need to make invoices in Chinese Renminbi 元. I have no problem getting 元 to show up with your basic online invoices, however when these invoices are printed as PDF the 元symbol becomes ▯

Is there anyway to rectify this? thanks

https://wordpress.org/plugins/sprout-invoices/