It sometimes appears that I have to write some official documents from some entities, especially for admin stuff. However, those letterheads are often provided in some proprietary format with which I do my best to make as few interactions as possible. Especially, I feel more comfortable editing some LaTeX instead of writing it in libreoffice (for instance).
Thus, thanks to Nicolas Trotignon who found a simple workaround, here is a small trick to do it. As a preliminary, you have to obtain a pdf version of the letterhead (to do so, I'm using online office 365 as it works in Linux, yes, shame on me).
Once this is done, the core of this method is to use the wallpaper package to embed the letterhead as a background image, and then play with margins to make it look nice.
Here follows a minimal example with the corresponding output. Replace background.pdf with your (empty) letterhead of course.
This can also be adapted to other classes, such as letter.cls.
\documentclass[a4paper]{article}
% Design and encoding stuff
\usepackage[utf8]{inputenc}
\usepackage[UKenglish]{babel}
\usepackage[T1]{fontenc}
\usepackage{libertine}
% The main trick
\usepackage{wallpaper}
\CenterWallPaper{1}{background.pdf}
% margin
\usepackage[inner=4.5cm,top=6cm]{geometry}
\begin{document}
% blabla
Hello, World!
% …
\end{document}