Learn Spanish (Latin America): Necesitamos los documentos, pero los enviamos mañana. - We need the documents, but we are sending them tomorrow.

This second los means "them". It is a direct object pronoun replacing los documentos (the documents) so we don't have to repeat the whole phrase. Because documentos is masculine and plural, we use los.

Why does the sentence say los enviamos instead of enviamos los?

In Spanish, direct object pronouns like los must be placed directly before the conjugated verb. While in English we put "them" after the verb, Spanish puts it before, essentially saying "them we send" (los enviamos).

Why are both verbs (necesitamos and enviamos) missing a subject word like "we"?

In Spanish, the verb endings tell you who is doing the action. Both necesitamos (we need) and enviamos (we send) end in -amos, which is the present tense ending for nosotros (we). Since the ending already makes it clear, we don't need to include the word nosotros.

Why use the present tense enviamos (we send) if the action is happening mañana (tomorrow)?