Hi,
Unfortunately, we didn't find a font which works for any UTF8 character and the library we use to convert the HTML of the invoice into a PDF doesn't provide that. We already added several fonts to improve the compatibility with some characters.
Regarding the issue with the footer, I was indeed able to reproduce the issue on my end. It's actually a bug which is in the library we're using to convert the HTML of the invoice into a PDF. It was reported years ago here:
github.com/spipu/html2pdf/pull/2
I've replied there with some indications to simply reproduce the problem. Hopefully, someone knowing the inner workings of the library can provide a long term solution.
In the mean time, what you can do is to open the file plugins/hikashop/attachinvoice/vendor/spipu/html2pdf/src/config/tcpdf.config.php and change "helvetica" in the line:
define('PDF_FONT_NAME_MAIN', 'helvetica');
to the font you've selected in the font setting of the plugin so that it will use the same one for the footer.