Special characters, such as “€” don’t show properly inside invoices

Special characters, such as “€” don’t show properly inside invoices.

This happens because your PHP install lacks the iconv() function, most likely because PHP has been compiled without iconv support.
You have to install and enable iconv in order for this to work. If you are using CPanel, then just recompile PHP with iconv() support, otherwise you’ll have to google how to install it depending on your linux distribution.