Open LETTERHEAD File
LETTERHEAD format files are internal files used by various post and page layout programs, such as Adobe InDesign.
About InDesign
Adobe InDesign is an application for designing and laying out pages and is also used for desktop publishing.
InDesign is produced by Adobe Inc. and is useful for creating works such as posters, flyers, brochures, magazines, newspapers, presentations, books, and e-books.
LETTERHEAD files are XML-based files that contain header settings for letters and envelopes, designed by InDesign.
The LETTERHEAD file is not designed to be opened manually by the user, in fact, it is generally not directly accessible.