It is likely that Mathematica notebooks (.nb) can be just simply exported as PDF documents right inside the main program using the FIle ►Save as.. function, which is likely what most users are looking for when they are searching for nb to pdf conversion. Unfortunately, we do not know about any 3rd party program that could convert nb to pdf with Mathematica notebooks in this manner.
There seems to also exist other file types that also use nb extension for their documents formats, but we have no particular information about these. But is likely that their respective programs should also be capable of PDF export or print via virtual PDF printer.Open NB file Open PDF file
Math visualisation and computing program