I see empty boxes ([][][]) instead of symbols in PDF
Default font (used by PDF library) doesn’t support all UTF8 symbols.
So if you use Chinese, Arabic, Cyrillic, Hebrew — you must install new font.
Please, use this way.
1. Download zip from https://freefontsdownload.net/free-st-gigafont-typefacescode2003-font-152444.htm
2. Run ftp client and create folder “fonts” inside active theme
3. Upload file Code200365k.ttf to folder “fonts”
4. Add following PHP code to functions.php