<style>
  @font-face {
    font-family: 'robotoslab';
    src: url("file://#{Rails.root.join('app', 'tmp', 'fonts', 'RobotoSlab-Thin.ttf')}");
  }
</style>

h5 {
    text-transform: uppercase;
    padding: 10px;
    letter-spacing: 0.2px;
    margin-top: 50px;
    font-family: 'robotoslab';
    font-size: 10px;
  }

Rails.root.join('app', 'tmp', 'fonts', 'RobotoSlab-Thin.ttf')
=> #<Pathname:/home/vamsi/code/certificate_generator/app/tmp/fonts/RobotoSlab-Thin.ttf>