Thanks for your reply and the links. I did a Google search before coming onto the forum and got pretty much the same results.
The third party utilities found in the first search are great for adding comments, graphics etc to a pdf, along the lines of editing in Word with track changes on. However, they don't actually allow me to compare old and new versions of a pdf and highlight the differences. The only ones I've found that purport to actually "compare" two pdf files are similar to Word's synchronous scrolling - they simply allow the user to move through both files side by side on the screen. The user then has to play "spot the difference".
Adobe Acrobat XI will do it - not perfectly but not too badly. Most of the documents I'd be comparing are in tables. I've discovered through the trial version of Acrobat that if the column width changes, and some words in the first line of a cell are now pushed onto the second line, this is flagged as a deletion and an insertion, even though nothing has really changed. I could live with this, but since I don't need anything else that Acrobat has to offer, I was hoping not to have to pay $200+. Ok, call me a cheapskate