My apologies - it was indeed the "Page Layout" page I was referring to. I'd entirely overlooked the fact that the page had a scrollbar, and I had to scroll down to see the fields for the right and bottom margins! I've now set sensible margins, and the converted book looks excellent.
Many thanks (as always) for the help.
In case anyone's wondering why I want to do something as strange as convert a perfectly good AZW3 book to PDF, it's because this is an Ancient Egyptian grammar textbook, which has exercises in the text, and an Egyptian dictionary at the back of the book, so there's a need to be constantly flipping between the two places in the book. I could do this with bookmarks in the Kindle App, but it's an awful lot easier to do in the "GoodReader" PDF app on my iPad which has a tabbed interface, and allows me to view different places in the same PDF in different tabs. Hence the desire for a PDF version of the book
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