Hi,
I suppose you could modify Unoconv converter so that it converts images in HEIC format into PDF.
For Moodle 3.9, in the function start_document_conversion defined in moodle/files/converter/unoconv/classes/converter.php, you may want to call an external command, say, imagemagick to convert $filename from heic to jpg format before exec($cmd). You may also want to modify the function is_format_supported($format) so that it returns true for 'heic'.