I have discovered a strange occurrence when "printing" to Adobe PDF.
Blank pages appear in the PDF that were NOT in the original Word document. At least they were not visible. This has happened before, but at least it is easy to remove unwanted blank pages from a PDF. But I wonder if that has some influence on what is happening?
I have finally taken this Word document and split it into two: one document for the front matter, and a second document from, and including, the table of contents. That stopped the very strange page jumping. So I can combine the two documents into one single PDF. This should not be necessary. I have had no trouble doing all of this with other Word to PDF jobs before, so this is an aberration, and I believe that it is a bug in Word, which would not surprise me.
The blank pages popping up in a PDF, that were NOT in the Word file, is a mystery.