Fonts must be supplied, embedded or outlined in artwork to ensure that they print correctly. A common problem with supplied artwork is when fonts are not packaged correctly, and if not rectified these will drop out of the printing. Your flight check software will pick up these font errors.
If you supply a PDF to a MKM Signs, please embed the fonts!
Sometimes if a font is licensed a PDF flightcheck might cause the error - "Licensing Restrictions". This means that even though the font has been embedded, it is not licensed. The only way around this is to create outlines and recreate the PDF. Sometimes you can open the PDF in Illustrator and do this.
If the artwork is supplied in InDesign, Illustrator or Corel, ask them to Create Outlines or 'Curves'. (And when doing this always save the 'outlined' file separately so the text can be edited later if necessary).