How can I load a custom twig template depending on a view mode?
Now the new theme hook is the same as the base hook. You need to declare the more specific theme suggestion as hook:
function mymodule_theme($existing, $type, $theme, $path) {
  return [
    'pdf_report__new_style' =&gt; [
      'base hook' =&gt; 'pdf_report',
    ],
  ];
}