Special Characters Become Question Marks "?" in PDF Printing
DOMPDF in CiviCRM have a trouble with special characters UTF-8. We have this same problem in Poland. I've create a repo with solution: https://github.com/ngo360/civicrm.dompdf.utf8

For apply this change go to path: path/to/civicrm/vendor/dompdf and delete all files. Then upload files from repo. I think is resolve Your problem.