Hi Josede,
thank you for your message!
You may copy/ edit the template of generated-invoice.php file from /wp-content/plugins/flexible-invoices/templates/invoice/generated_invoice.php and add its copy to your theme directory /wp-content/themes/[your-theme]/flexible-invoices/invoice/generated_invoice.php
You may add additional info e.g. footer like this: https://wpdesk.me/tb/9CrnTS4WHK.png
The Flexible Invoices WordPress plugin supports Latin characters and the DejaVuSans font by default.
The plugins use the mPDF library and the font to issue invoices dynamically.
You could try to add ttf version of a font into:
wp-content/plugins/flexible-invoices/class/mpdf/ttfonts
In the upcoming version of the plugin, there will be a possibility to change/ choose the font. I may inform you about that update if you want to.
Let me know if you have any further questions.