Using this font in your PDF templates, you will be able to render PDF invoices that contain beautiful Chinese characters, which would not be available with Helvetica font.
Unzip the file. It contains 3 files. You must place all 3 files inside folder /components/com_invoices/helpers/dompdf/lib/fonts/
Then, in your invoice template, you just need to use the FireFly Sung font. A good way of doing so is by starting the template with a DIV forcing this font:
div style="font-family: FireFly Sung; font-size: 12px;"