they can only be edited with the full version of Adobe Acrobat, which costs quite a lot, (£200 plus I believe)as opposed to the Acrobat reader, which is free...
However, depending on what you want to do there are other ways to get to the information...
There are "text select" and "graphics select" buttons on the reader toolbar... you can use these to select then copy and paste text and graphics into another document, eg a "Word" document.
Some OCR, (Optical Character Recognition), software, (cheap versions usually come bundled with scanners), can "read" .pdf files and save the results as a "Word" document.
There are some pdf converters, which do the same as I've just mentioned. They have been on magazine coverdiscs in the past, so they're not expensive. I can'r recall exact name off the top of my head... type "pdf converter", with quote marks into google and see what comes up.
There is an alternative pdf reader called "Foxit", whihc has a free version. Using this you can "overtype" and print out the results, but not save as a pdf.
I hope this helps,