When an assessment question includes special characters (e.g. Greek letters) they do not appear in the PDF export. However, they do appear to students when they are taking the assessment and the special characters also appear in the "Print HTML" option, but not the PDF.
STEPS TO REPRODUCE:
Create an assessment and add a question including a Greek letter in the question or answer.
Choose "Download PDF"
Either a damaged file or a PDF that omits the Greek letter.