You can customize your PDF Invoice font by add the bellow snippet to the beginning of Liquid template.

<link href="<--your-font-url-->" rel="stylesheet">
<style type="text/css">
body {
font-family: '<--you font name-->', monospace;
}
</style>

For example
<link href="https://fonts.googleapis.com/css?family=Roboto+Mono:100,100i,300,300i,400,400i,500,500i,700,700i&display=swap&subset=cyrillic,cyrillic-ext,greek,greek-ext,latin-ext,vietnamese" rel="stylesheet">
<style type="text/css">
body {
font-family: 'Roboto Mono', monospace;
}
</style>