Links to Media Library items not working – 404

Links on WordPress pages to PDFs, Word docs, and a PowerPoint presentation do not work.

Internal and external links to web pages do work. A link to an Excel spreadsheet works: it automatically downloads in the browser.

However, links to *one* PDF and *one* Word document do work — by downloading to computer, not by opening in browser. They had a full link path including https://, while the others had a path that began with the domain name. (I’m using the WordPress HTTPS plugin to satisfy security certification requirements.) But when I change a non-working PDF and a non-working Word link *on that same page* to include the https prefix, they are still 404.

Other information:
I’ve been developing a Custom Post Type for a document repository. Because the rest of the site is only Pages, I figured I could mess with the Permalinks setting. Maybe not. I’ve changed it back to the original Permalinks setting — /%category%/%title% — but that hasn’t fixed the problem.

I see the files in the Media Library, but I can’t get to them by pasting the URL into a new browser windows. I can, however, FTP the files, which are in the proper folders for the URLs, to my computer and open them on localhost. I wasn’t the one who uploaded the files, so I really am getting them via FTP.

I’ve looked at other posts about 404 document links, but they are old enough that they don’t seem to apply. Any ideas about what may be going on?