Pretty much any document or graphics file can be exported to PDF with virtual PDF printer. The only thing you need, is some software that can open your .ptb files and print them to PDF. Many different .ptb files exists and it is likely that at least some of them can be saved as PDF files.
One of such .ptb files that can be exported this way are guitar scores from Power Tab (music sheets), so perhaps this what users searching for ptb to pdf conversion are looking for.