Well if it's going to be for print, such as a printed magazine, a magazine/newspaper ad, etc, and a graphic designer wants a certain font on there that not all computers have, PDF is the way to go.I personally prefer HTML (simple one, not Microsoft Excel-export).
The user can choose his own font, it is fast, it can be parsed to be converted in any format.
Don't know how flexible the PDF-converters are already concerning this.
Mark
If it's on line, I'd say HTML is the way to go.