As you probably know, a PDF document is an Adobe Acrobat file and it's not editable. If you want to be able to edit the document, you must convert it back to some format that you can edit. It's always best to save the original document, so you can recreate the PDF if necessary, but if you lose the original document, there are lots of different ways to convert a PDF back to editable format. If you own a copy of Adobe Acrobat (not the free Acrobat Reader, but the full version), it can do this job. And for most text it does an excellent job. The process is called OCR (optical character recognition) and almost any software that includes this feature can convert from PDF back to editable text. One of Ken's favorites is the Mac app, PDFPen, which includes OCR functionality and does a fine recognition job.
If you search the web for "PDF to Word", you'll find several tools online that will do this conversion for you as well. All isn't lost if you can't find the original document, but clearly the best solution is to hold onto the document from which you created the PDF file, so if you need to make changes you can edit the original and then recreate the PDF.