As far as we know, it is not possible to directly convert Safari .webarchives to PDF format. However, you can open the .webarchive file in Safari web browser and export / print the page saved from within it as a PDF document. You can download Safari for free from Apple's websites.
This will create a .pdf file that contains the opened webpage, including pictures and other elements, as a document, essentially achieving webarchive to pdf conversion.Open WEBARCHIVE file Open PDF file
A default web browser integrated to Apple Mac OS X