Does anyone know how to edit text or add text to a document that has been
scanned?
A scanned image is essentially a *picture* of the text, not the text
itself, and it's therefore not editable.
If you want to edit the text, you need Optical Character Recognition
(usually abbreviated OCR) software. That kind of software look at the
picture, recognizes characters, and turns it into editable text.
There are several OCR products available: OmniPage and Abbyy Fine
reader are two well-known ones. Scanners often come with "Lite"
versions of such software, and if you look around, you may find that
you already have such a product.
I personally use the Lite version of OmniPage, which came with my
scanner, and it works well for me.