phpunit pdf tofu characters detection

I'm trying to design a phpunit test to detect tofu characters within a generated pdf. (If none of the fonts included in the pdf supports the language within the pdf, tofu characters will appear.)

First, I tried to get the pdf text, however the getText method always returns the correct unicode text, even if tofu characters are seen within the pdf.

Second, I've considered listing the available fonts and simply reviewing that all the required fonts are present.

$parser = new PdfParser();
$document = $parser->parseFile($pdf);

$fonts = $document->getFonts();

foreach ($fonts as $font) {
    $font->getDetails();
}

Would anyone have a better approach to suggest?

Start with the issue's PdfParser parseFile call and the getText, getFonts, and getDetails entry points. Determine how a PHPUnit test could distinguish rendered tofu characters from the extracted Unicode text, then define a reproducible PDF fixture and a passing detection test as the completion criteria.