How to "repair" a PDF that opens in Acrobat Reader but not Preview

I have a PDF that won't open in Preview; it gives this error message: However, it will open in Acrobat Reader, but cannot be resaved, or saved with a new file name from Reader: As I can see all the pages (or most of them at least!) from this PDF, there must be some way to get them into a file that Preview can open. I can do this by copying pages from Reader one-at-a-time, but the PDF has 500 pages. What is the easiest and cheapest way to export all these pages, and create a new PDF, from Acrobat Reader? **EDIT:** I'd rather not have to sign up for online Adobe services...