Retypeset the letterhead and this time, save it in a format Word can handle. PDF is more for output and printing, rather than being useful for editing, and that is by design.
Usually, one would create a letterhead, along with style sheets, and save that as a template that one would load any time one wanted to use that letterhead. I do not know of any simple or reliable method of utilizing a PDF within a Word document - all methods would involve using Acrobat to bust out the letterhead, saving it as a graphic, then importing it into Word. This may be a non-trivial process, since Word uses oddball rendering that may make the graphic blurry, and that again, is by design.