Another surprise from my inherited doc set, though in general my predecessor didn't do a bad job: a paragraph style specifies a background colour (CMYK, process) to give white text on a blue background on the cover page. No problems on screen, otherwise I would of course have noticed earlier – but creating a .pdf unhelpfully drops the background so I end up with discreet/confidential white on white.
What's going on, and how I sort it?
Check your color definition for the color White (View > Color > Definitions). "Print As" will probably be set to "Don't Print". Change this to "Print As... Process" and set Overprint to Knock Out.