You can display this error, if you:
- will turn on the php error reporting in your php ini
- or will take a look at the error log on your server.
But it seems like on your site there is no php error, as I checked the pdf and it has correct header without any php error message, it seems like there is some problem with font or encoding (try to install other font, try to install helvetica font too - will not include the font, see:
https://www.phoca.cz/download/category/34-phocapdf-fonts
See the image:
Jan