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.

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Cheers,
Paul Edstein
[Fmr MS MVP - Word]