This artice shows how to extract all the certificates and key file from .pfx file using openssl and remove any password if required.

I own a Synology NAS and it now supports the free certificates from Let's Encrypt.  The only problem with Synology's implimentation is that they only allow the web (HTTP-01) method of certificate verification.  This means that I have to map port 80 and 443 from my NAS and make it available to the Internet.  Yuch!   This is what I did to use Let's Encrypt and not open up and web ports to the internet.