how to edit scanned PDF

Preview only allows you to do annotation on PDF, so you can't edit a scanned PDF with Preview.

I would recommend you Abbyy FineReader OCR Pro and PDFElement, both support OCR recognition, results are good.

Choose one of the available PDF Editors, whether in the Mac App Store (e.g. PDFpen/PDFpenPro, etc.), or subscription to Adobe Acrobat Pro. Apple does not provide a PDF editor — not in Preview, or the Pages application.

You may find your ability to edit a scanned PDF are limited. Essentially, scanning takes a picture of a document. Unless you run it through optical character recognition, there won't be much you can do other than mark it up. The full version of Adobe Acrobat does have the ability to try to run OCR on a pdf. My experience is that it doesn't always work.

Which is it?

Scanned or Saved As?

They are not the same thing.