How can I having a site url obtain its certification in order to use it with some program I'm using that'll have to use it?

the page im talking about is: https://webpac.telepac.pt/detailTraffic/sapoADSL.html

I need the .CER file of that page, I tried viewing it using mozilla and it says which is it but it doesnt allow me to expert it to a .cer file..any suggestions? To acomplish what I need I have to really have the .CER file

Thanks in advance