OR the font they are using has licensing restrictions and will not allow it to embed. There is no system in Acrobat to warn you when you are view a file and it has substituted fonts for display. Remember when this all started, this was one of those big advantages of a PDF, then end user didn't have to have the fonts because the PDF would have embedded replacement fonts (adobe sans, adobe serif) to use if needed.
Matthew "LAMMY" Lamoureux
918 Printery - Ad artem artium conservatricem conservandam