Hello Howard,
Sorry for the trouble, as per the description above, Adobe Reader is not displaying a font as you are getting an error message, Is that correct?
Please navigate to Adobe Reader's Preferences from Edit>Preferences>Page Display>Select "Use Local Fonts">Click OK>Close the PDF and Adobe Reader.
Does the change in the settings brings any difference in the PDF? If the issue persists, most likely the problem is that the PDF file does not have "DavidD" font embedded in the PDF file.
To confirm that the font is not embedded within the PDF file, open the PDF file and press Ctrl-D. Open the Fonts pane and scroll down looking for "DavidD". Next, to the font name, it should not say (embedded). That would confirm that the problem is due to the font not being installed on your system.
You might have to contact the author of the PDF and have to ask to embed the font in the source file or the in the PDFMaker application.
let us know how it goes and share your findings.
Regards,
Anand Sri.