Why is 5.1.1 rom always "corrupt" ?

Hey guys, sorry I have been away for so long and am not updated... anyway, I have been downloading different 5.1.1 roms (samfirmware and sam updates) for my N910C and everytime I try to unzip, it says the file is corrupt...

Can anybody please tell me what has changed? Whay can't I unzip the downloaded files?

I only want the official rom, and am not rooting my phone for now.

Thanks