Hi,
Which version of the PDF invoice plugin are you using ? What did you select in the "font" setting of the plugin ?
In the code of the plugin, we use the function sefDefaultFont of the HTML2PDF library in order to set the font that you selected in the settings of the plugin. And it's then the job of HTML2PDF to include the font to the PDF. So I'm afraid there is not much we can do as we don't have control on this from the plugin's end.
Did you add custom CSS to the attachinvoice/invoice.php layout file where you change the font in CSS ? That might explain why it's looking for a different font ?