Learn Spanish (Spain): Adjunten la copia al documento.

Adjunten is a formal command given to a group of people (ustedes). For regular -ar verbs, you form this command by dropping the -ar and adding -en. The word adjuntan ending in -an is just a regular statement meaning "they attach" or "you all attach."

What is the infinitive (dictionary form) of adjunten?

The verb is adjuntar (to attach). When you give an ustedes command, you swap the regular -ar ending for -en.

If I were giving this formal command to just one person (usted), how would it change?

You would say adjunte. For a formal singular command, regular -ar verbs swap the infinitive's -ar for -e.

Why is it al documento and not a el documento?

In Spanish, whenever the preposition a (to) is followed right away by the masculine "the" (el), they must squish together to form the single word al. Since documento is masculine, we must say al documento.