I was actually able to do this on Windows 10.
Install Safari from here: http://www.techspot.com/downloads/4184-safari-for-windows.html (Click "direct download) and then right click the .webarchive file you're trying to open, point to "open with" and choose Safari. It'll open it, along with a print window.
I'd recommend selecting "Microsoft print to PDF" as the printer and saving it as PDF, but if you cancel the print dialog box it'll open in Safari just fine.
Crédito: Pedroelbee