If the "simple letters" are done using PS (via LaTeX) you need at least GhostScript. If you're interesting in viewing your PS documents, then you need GSview as well. The GSview download page states that:
GSview is a graphical interface for Ghostscript. Ghostscript is an
interpreter for the PostScript page description language used by laser
printers.
To that extent, you have to set the PS interpreter (GhostScript in this case) in GSview in order for it to work, otherwise GSview won't be able to display any PS content:
