En est un pronom qui fait référence à ce dont la lecture est facilitée par la présentation en question, un livre, une affiche, une notice, etc.

En is a pronoun that refers to the writing material which has its readability improved thanks to the presentation the sentence is about, whether a book, a poster, or whatever document.

Without en, the sentence might have been:

La présentation aérée permet une lecture facile du document.

The most literal rendering in English would be “of that/it” or “of X” (but placed at the end of the sentence). Often, though, it isn’t translated directly. It is completely different from “en” meaning “in”, so don’t let that throw you off. On an interesting side note, Italian has a word used almost exactly the same way (“ne”).

1. With “en” pronoun

Sa présentation en permet une lecture facile.

The above sentence could be translated by:

Its presentation makes it easy to read

2. Without “en” pronoun

Sa présentation permet une lecture facile.

The above sentence could be translated by:

Its presentation allows easy reading