By Default WooCommerce Pdf Vouchers uses Helvetica fonts. If you check the option "Enable characters support" from a global setting page, It will use Freeserif fonts which will work for most of the languages.
Suppose you are using a language which is not supported, please download Multi Language Characters support plugin. Once you activate this plugin it will add a Font selection option as mentioned in the image below:
This plugin contains following fonts:
If you do not find your font name from the all the available fonts above, please select Custom Font option from the dropdown. You will be able to see a textbox where you can enter the name of your custom font. You need to put your custom font in wp-content/woocommerce-pdf-vouchers/fonts
folder.
You can override the font via voucher template as mentioned in the image below, please leave it empty if you want to use font from global setting.
To change font for generated PDF you can add this code snippet to a custom plugin or your child theme's functions.php by replacing 'freeserif' with a name of your font.