Official comment
Unfortunately, Firefox and Chrome no longer support the most common way to do this. Previously, you could have the link itself indicate the file was to be downloaded, but modern browsers view this as a security threat and only allow downloads from the same domain.
You could store the file on your website, and the download link would work. But the browsers will block any file from downloading automatically if the file is from another domain, like files in your Ontraport account. The user has to specifically tell the browser to download the link, either by right clicking the link and selecting "save as" or opening it in a new tab and then downloading it.