GitHub has a file size limitation. One needs to download the files with the .iso.zip and the .iso.z01 and the .iso.z02 extensions, into the same directory before extracting them – they should extract and merge to form the .iso file.
sudo pacman -S 7zip
But try here instead.
These link to the uncompressed ISO. 
In fact, this is the better choice.
Download the checksum file as well (.sha256) to verify it.