My document contains Unicode characters above U+FFFF. It looks fine in the editor but when I export the document as PDF the astral characters instead display as two boxes, probably surrogates. How do I make LibreOffice handle all characters correctly?
What operating system and what PDF program are you using to display your pdf file, and is it on the same system as you created the file? If you are using a LibreOffice font to create the document, you must have an equivalent font available with the used characters defined. It sounds as if you pdf program does not recognise your higher unicode characters. Each character then being displayed as two or three ordinary ISO-8859-1 characters.
You can export .pdf files including the fonts your are using. This makes them rather larger but allows you to check a potential problem… Peter