Learn Italian: I documenti italiani sono pronti. - The Italian documents are ready.

I is the masculine plural word for "the". The singular noun documento (document) changes its ending from -o to -i to become plural. Because documenti is masculine plural and starts with a regular consonant (d), the singular article il changes to i.

Why do both adjectives, italiani and pronti, end in -i?

In Italian, adjectives must match the noun they describe. Since documenti is a masculine plural noun, the singular adjectives italiano and pronto both change their ending -o to -i to match.

Why does italiani come after documenti?

In Italian, descriptive adjectives (like nationalities) usually come right after the noun they describe. So, instead of "Italian documents", it is written as documenti italiani (documents Italian).

I thought sono meant "I am". Why is it used here?

The verb essere (to be) is special because the form sono is used for both "I am" (io sono) and "they are" (loro sono). Here, because we are talking about "the documents", sono means "they are".