Apparently it was created on another machine with a different installation of systems fonts. Try picking another font. See the Adobe explanation from their support site:
Use the TouchUp Text Tool to change the font in order to edit the text.
1. Choose Tools > Advanced Editing >TouchUp Text Tool.
2. Select the text you need to edit with this tool and right-click (Windows) or command-click (Mac), and then choose Properties.
3. In TouchUp Properties dialog box, select the Text tab.
4. In the Font drop-down list, select a Font consistent with the current font you are using (these fonts listed are available on your system). You can choose to embed the font into the document by checking the Embed check box, or simply subset only the characters you'll add by checking the Subset check box (which keeps the file smaller).
5. Click Close, make your edits, and then Save your document.
Background information
If the fonts in a PDF file are not the same as those available on the current machine on which the PDF is being viewed, or they are a different version of the font than available on current machine, you may not be able to edit text in the PDF. Changing the text to a font available on the system will allows you to edit it the text in the PDF.
Let me try to explain this. Apparently the font is somewhere in Acrobat 5 but not in Acrobat 6. I have both programs loaded on the same WinXP Pro computer. The font is Helvetica Neue Bold. I have no special attachment to this font, but it just is so irritating that Acrobat 6 (I even tried 7) can't recognize the font.
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How do I add System Font in Acrobat 6.0?
All or part of the selection has no available system font. You cannot add or delete text using the currently selected font.
I have the full install Acrobat 6 Pro version but can't do Text TouchUp on an Acrobat 5 file.
Any idea why?