Thanks GOULI, so far not the answer. So frustrating. It's taken me several years to be in a place where I can start writing again (Mind, not house) and this hits me in the first minutes.
I know I haven't set a password and it worked perfectly for a couple of years. The net generally seems to show this is being experienced by others*. This file labelled simply, BIG, is my entire book in one file. I have others with each chapter in its own file, and another called Numbers, where the pages are numbered. Other files have name of me and the book top right corner for submission etc., etc. Only this pdf has the 230,000 words in one lump.
It was astonishing how hard it was to simply paste the chapters together. So simple in Word Perfect, but oddly entwined with other things in Word.
Word had features I couldn't be without, but the difficulty with simple things like numbering pages seemed bizarre after WP. A powerful tool once under control.
*One poster mentioned erasing: homediretcory/library/preferences/com.adobe.reader.plist and all will be well. Of course, I can't find that file.
Tomorrow's another day.