Pagehand is a Mac OS X Program which allows you to save your documents as PDF and later edit them again. It can not open all PDFs though, if they were created with different programs.
PDFpen looks promising (from SmileOnMyMac, creators of TextExpander), although it's relatively expensive."
You don't say what sort of editing you need to do. The standard Mac Preview app has tools for annotating PDFs -- drawing circle, boxes, arrows, highlighting text.
