Perhaps you could provide a link to the document you are having problems with?
Alternatively, if all you want is the letterhead that's on the PDF, maybe it would be easier to simply create a "blank" letterhead form from scratch in Designer. You can place pictures, text, etc. on a blank page in Designer. If you place things on the Master Page, they will show up on every page in your document. Once you've created your blank Designer letterhead, save it, and then use copies of it as your base for creating new forms.
Hope this helps.
Stephanie Legault
Software Developer
Adobe Systems