Finder - how to download a file from iCloud but prevent finder from opening it straight away?

Just don't click the filename, but click the cloud icon with the download arrow to the right of the filename. This will download the file without opening it. Clicking the filename indicates that you want to open the file.

Hey there Gedas_S,

Thanks for stopping by the Apple Support Communities.

You can download files from your iCloud Drive by using a control-click on the file and selecting "Download Now" from the pop-up menu. The Apple Support macOS User Guide Work with folders and files in iCloud Drive - Apple Support describes the specifics below:

In some cases, items may be stored in iCloud Drive but may not yet be downloaded to your Mac. 

1. Control-click the items that you want to download (indicated by the In iCloud status icon ).

2. Choose Download Now.

After the items are on your Mac, you can work with them without an internet connection.

Best!