Hello,
can we do some test? Go to:
Invoice Configuration -> Invoice Template
and there is in header {contact}
Modify it to {contact} - [all_special_polish_characters]
Use Free Sans (UTF-8) and print new PDF invoice. If there will be some characters missing, can you send me printscreen example as attachement here? Maybe better will be real pdf packed to zip archive.
I tested Free Sans (UTF-8) on my local site and it prints me all czech and slovak characters. I don't know all polish characters, but 'ć' and 'ó' was printed as well.
If there will be still any problem, it will be probably not because of fonts and I will have to consult it with development.