Bob.
I don't think I'd try to mess with it, personally; PDFs weren't designed to be editable (though it can be done with third-party apps, as others have said) but were meant to be a 'digital print' of a document, hence it's popularity as a proofing format. If this is a 'proof' of a to-be-printed brochure, then IHMO you'd be better off bringing the spelling errors to the attention of the designers/printers and letting them correct and re-proof it. If this is a PDF brochure, then again, I'd bring the errors to the attention of the designers and have them re-do it.
Cheers, Pete