Your further descriptions confirm what I posted before - your document has odd/even page Section breaks. Since you're using those to ensure the page numbering follows the correct sequence, Word has to insert a blank page at print time to ensure the odd & even pages always occur on the same side of the paper (imagine a duplex printer). For a PDF, the same convention needs to be observed.

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

I tried several things
1. I got rid of all odd-even page breaks, substituting continuous breaks, and then blank pages before the break if needed. That did not help.
2. I started this document with 25 sections (chapters), and each section ran page numbers from the beginning, but without a page number on the first page of a section. The problem began when I started to put in the front matter. I wanted the main body of the book to have its own page numbering, and that is normal. Then I went back and instead ran page numbers right from the very first page, without changing any section breaks. That worked! No more pages jumping around. But that is not how I wanted to run page numbers. So I will just work with two documents. That at least works.
I have done this work before without these problems. But I think you are right, that there is something with section breaks. Somehow running page numbers right through all sections gets rid of the problem. The page numbers are in the footers, so maybe they need to be unlinked in some way. I did go back into the document and unlinked all headers and footers. That had no affect whatever.