Convert BAK to PDF

BAK files are pretty much just renamed backup copies of the original file, which means that if your file was PDF, you can simply rename it back and open using PDF Reader. If that's not the case, your .bak file is not PDF backup and there is most likely no way how to convert it.

You might also want to check out some file analyzer software to determine what kind of .bak file it is and whenever it may be even exported as PDF document.

Simply put, it is impossible to convert BAK to PDF directly.
Thus, there is no so-called bak to pdf converter or a free online .bak to .pdf converter to be found.