Thank you, Colin, for your reply.
The Upload directory path is set as you say, and I can see the PDF file in the "<Category>" file, which is "Sermons".
The Download and Category each have their id set.
The Global Config. settings are as you describe.
Something more that I have noticed:
The failing link with the "download" path segment in it is actually the Link for the download title in the Article generated by the Content plugin. This is gives the error page. The "Download" button itself is the one with the "send" path segment which successfully gives the file itself. I am not sure what is supposed to be in that title link. Might it be the Categories link? Or the detailed information page for the download file?
I've also noticed this in the backend in the jDownloads component:
The "Link" link shows the error in the backend Categories page in jDownload Component:http://localhost/canonsregularRedotwo/index.php?option=com_jdownloads&view=category&catid=2
The "Download this file" symbol link shows the file successfully in the backend Downloads -> Edit Download page for this download in jDownload Component:http://localhost/canonsregularRedotwo/administrator/index.php?option=com_jdownloads&task=download.download&id=1
I'm wondering if it is an install error, something not enabled, or else I still have something misconfigured.