Resolved with help from Mathematica Support. (Embarrassing but necessary.)
It seems that the Mathematica Print command was set to print Pages 1-3 (unbeknownst to me) - and this becomes a property of the Notebook, until changed.
Thus SaveAs to .pdf creates only a 3-page .pdf.
Changing Mathematica Print command back to Print All FOR THAT NOTEBOOK enables SaveAs to .pdf to create .pdf of entire document.
This was confusing because a test Notebook to .pdf worked fine, because, obviously, it had defaulted to Print All.