If the downloaded file ends in .download (Safari) or .part (Firefox) or .crdownload (Edge/Chrome) then the file is either still downloading (and you need to be patient whilst it completes the download) or incomplete, if the download has stopped/crashed or you have had a Network Error (this indicates a partial download) and the file you have will not work. It often won't UNZIP as it isn't all there, and if it does then some (or all) of the files inside it are incomplete/truncated.
To do this if you unzip the ZIP file downloaded, it should contain more than a single file (usually 3 or 4 files).
If it only contains one file, then the download is incomplete and you will need to download the file again, making sure that you follow the guidance here!
If you want to run a test on the downloaded ZIP file, please follow the guidance below (for the relevant ZIP tool/application):
If you are using 7Zip, you can Test the archive to see if it is complete and valid (just click on the 'Test' icon when the ZIP is open in 7Zip), see below.
You will see the results of the test, like below:
If it says there are no errors, then the ZIP file (and its contents) are complete (valid), if it says there are errors, then it is incomplete.
WinZip
You can do the same with WinZip (Unzip, Diagnostics),
You will see the results of the test, like below:
If it says there are no errors, then the ZIP file (and its contents) are complete (valid), if it says there are errors, then it is incomplete.
WinRAR
You can do the same with WinRar (Test).
You will see the results of the test, like below:
If it says there are no errors, then the ZIP file (and its contents) are complete (valid), if it says there are errors, then it is incomplete.
There are many other ZIP handling applications, but these are the most common ones. If you are using a Mac, Finder will normally automatically UNZIP the downloaded ZIP file; either after it has downloaded, or when you double-click on it.