De or het geleidebrief?

The correct Dutch article is:

de geleidebrief

How to use geleidebrief

the geleidebriefde geleidebrief
this geleidebriefdeze geleidebrief
that geleidebriefdie geleidebrief
our geleidebriefonze geleidebrief
your geleidebriefjouw geleidebrief
every geleidebriefelke geleidebrief
which geleidebrief?welke geleidebrief?
a big geleidebrief / the big geleidebriefeen grote geleidebrief, de grote geleidebrief

Dutch words ending in geleidebrief

vrijgeleidebrief

In compound words, the last part decides the article. See the rule