Thanks for the response. I looked at the Altium Designer Smart PDF Wizard, and its configuration just specifies what is to be included, and how the results should be organized; there isn't anything like a "Make the PDF More Searchable" checkbox.
After trying a half dozen PDF viewers for the Mac, I've found that some do better than Preview at searching these, and others don't, with no real rhyme and reason. My best guess is that the standard macOS preview isn't highly-tolerant of somewhat eccentric PDFs, and Altium Nexus generates PDFs that look and print fine, but are internally somewhat eccentric, which makes them harder for PDF readers to search them.
For now I'm switching to Acrobat Reader, which seems to search the PDFs well, although it has some other eccentricities. Such is life...
Thanks again,
Dan