How to get rid of "Deleted Hidden Page And Image Content"

You need to re-save the document to finish the process. If you save and re-run Examine and the items are still there, then there may be a structural issue with the PDF that prevents them from being removed.

Thanks, I saved then re-run Examine and Deleted Hidden Page And Image Content is still checked. How would I find out if there is a structural issue with the PDF?

In Preflight there's a profile under PDF Analysis called "Report PDF syntax issues" that checks the file at low level for anything hinky. You can also drill down into the tree shown in the Examine Document sidebar until you get to the bottom entry, "show preview", which will display the offending thing on the page - it may then be obvious why it isn't behaving; it may not. Without seeing the PDF in question we can't of course help a great deal, but if the file's confidential you can't share it with us anyway.

Examine is a lot more reliable in Acrobat 9 (after all, it's designed for lawyers) but some PDFs have very strange combinations of bounding and crop boxes, and Acrobat may be getting confused as to what's actually "hidden" and what's not. If for example a character in a text box is half on the page and half not, is it hidden? Your guess is as good as Acrobat's, but it can't cut an individual character in half, as that's not how font outlines work.