I have scanned a document and recognized text. There's a typo that needs to be corrected but TouchUp Text tool will not make the change.

I select the character to be edited with the TouchUp Text tool and when I type the replacement character I get a this message:

"Warning! Since the original font is not available, a substituted font for editing is used. Any changes to the text of the original font will place a new font dependency upon the document."

The fonts listed in the document are fonts on my system (Arial, Helvetica).

Scanned at 600 dpi, text/line art, on Epson scanner, G5, OS 10.4.8, Acrobat 8 Pro.

Why is this happening? Why can't I edit the text after scanning and OCR?

Ken