To show own fonts you need to first add .ttf file to application. So open your App_Data/Pdf folder and copy your font to
Next open your nopCommerce Configuration/Settings/All Settings (Advanced)
Find setting pdfsettings.fontfilename and change value to your font name:
To show how it is working will add just only one string. That will affect all invoice, so will be empty invoice except that string added by me.
Please remember to check “My language has diacritical marks” setting at the first tab of plugin configuration.