Learn Portuguese (Portugal): Os documentos estão impressos?

Because here, the past participle impresso is acting as an adjective describing Os documentos. Since documentos is a masculine, plural noun, the adjective must match its gender and number, becoming impressos.

Is impresso an irregular past participle?

Yes! The verb imprimir (to print) has a short, irregular past participle: impresso. When you use a past participle as an adjective to describe a state, like we are doing here with the verb estar, you use this irregular form.

Why do we use estão instead of são here?

We use estão (from the verb estar) because we are talking about the current physical state or condition of the documents. They are in a printed state as the result of an action. The verb ser (which would be são) is used for permanent, inherent traits.

Why isn't the word order changed to make it a question?

In Portuguese, you don't need to swap words around or add helper words to ask a question. You just use the exact same word order as a normal statement (Os documentos estão impressos) and add a question mark. When speaking, you simply raise your voice pitch at the end of the sentence.