In order to edit a PDF, you would need Acrobat and not Reader but of course it's best to edit the original document then create a new PDF from that (also requires Acrobat to create the PDF).
As far as repairing a corrupt document it depends. For the most part, it can't be done. What is wrong with it (are you getting errors...etc)?
Well, the computer says it cannot open the document and causes the program to stop running. Thanks!