You cannot edit a scanned PDF document. It is basically a photograph of a document, just in PDF form.
You might - just might - be able to convert the document in to a text document like Word, but there will be a lot of errors in it if it was able to make the conversion. Letter combinations like "cl" will become "d," or a "rn" combination will become an "m." Things like that.
I have scanned documents to a PDF and then converted it to an editable Word document, but it will not have any formatting, really a pain. I have to play with my system every time I try it, but I believe if you open the document, you can click on file, then SAVE AS and select text. Then close all that and open the txt document. Like I said, it will lose all formatting, but the text will be there.