Most likely the file no longer exist in the site.

What is in the context menu if you right-click such a file ?

You can also check the history as download are special history items marked as a download.

The files do exist on the site. the issue is that they are not served over a secure connection. The company says they are safe to download. I tried. This is the result. It is permanently stuck unless I clear all downloads which I can't do without about a week of filing.

I think you only other choice is to re-download them again. Unless the site has a time out link that can also happen if the connections is too slow.

I'm OK to not download them. I have cancelled my subscription to that Business Books service. What I want to do is stop the downloading effort in case it is putting a load on anything, but without deleting all my downloaded files before I can file them.

Those are from the looks incomplete so the pdf will not format and nor will you be able to view them if that is the case. Maybe your cancellation is why your downloads stopped since you no longer have access to those files. You can't continue a download if your subscription is cancelled. That's not how paid sites works.

See also:

• 1686756 - Insecure downloads with https-only enabled cause zero-byte files to be downloaded.
• 1754920 - A blocked insecure (mixed content) download that we intend to open once downloaded, results in a 0-byte file and download panel oddness

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https://https://bugzilla.mozilla.org/page.cgi?id=etiquette.html)