If your document has odd/even Section page breaks, that could account for the behaviour.

__________________
Cheers,
Paul Edstein
[Fmr MS MVP - Word]

Thanks for the suggestion. I think my problem is a more deep seated, and pathological problem. I have spent over an hour trying various combinations of section breaks, and that has no influence at all.
I did, however, discover that the page numbering does have an affect. If I let page numbering run unbroken from the start it has some affect. But that defeats the whole idea of using section breaks. There seems to be some bug in this at a particular place in the document. This only happens between the first and the second chapters. The way the pages flip over when scrolling is very bizarre. I think I will try to do this in Pages instead. I will report back if or when I find a solution.

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.