We are so sorry to hear that this is happening to you- this appears to be an isolated Mac issue.
If you are getting a file ending in .cpgz when you are trying to download the original .zip, alter the file extension of this newly created .cpgz to be a .zip. Double click on then .zip and it should extract properly!
We would also recommend changing the web browser and then try the download again.
If you are still having an issue, please do get in touch!