Because the output of any scanner is an image you will have to take the image out of the PDF and then, in an image format, use an image editor (Photoshop, Photoshop Elements, etc) to effect the changes. Of course doing this may become a bit tedious.
If you use Acrobat XI or one of Adobe's online subscription services (ExportPDF or PDF Pack) you could OCR the image of text that came from the scanner and that got put into the PDF. Then export to a Word file. Then cleanup / edit as desired.
Be well...