Preguntamos si el documento necesita una apostilla.

Yes. Preguntamos is the nosotros form of preguntar, and for -ar verbs that form is the same in:

• the present indicative: we ask / we are asking
• the preterite: we asked

So, by itself, preguntamos is ambiguous. Context tells you which meaning is intended.

Examples:

• Preguntamos si el documento necesita una apostilla. = We ask / We asked whether the document needs an apostille.
• Ayer preguntamos... clearly means Yesterday we asked...
• Siempre preguntamos... clearly means We always ask...

Because Spanish usually drops subject pronouns when they are not needed. The verb ending already tells you the subject:

• preguntamos = we ask / we asked

So nosotros preguntamos is possible, but it is usually only added for emphasis, contrast, or clarity.

For example:

• Preguntamos si el documento necesita una apostilla. = neutral
• Nosotros preguntamos si el documento necesita una apostilla. = We asked, not someone else

Here si means whether or if in an indirect yes/no question.

So:

• Preguntamos si... = We asked whether... / We asked if...

This is different from:

• sí with an accent = yes
• si without an accent = if / whether

Examples:

• No sé si viene. = I don’t know whether he’s coming.
• Sí, viene. = Yes, he’s coming.

It is the same word, but not the same function.

In your sentence, si introduces an indirect question:

• Preguntamos si el documento necesita una apostilla.
• We asked whether the document needs an apostille.

A conditional si sets up a condition:

• Si el documento necesita una apostilla, la conseguiremos.
• If the document needs an apostille, we’ll get one.

So the word is the same, but the structure and meaning are different.

El documento means the speakers have a specific document in mind, one that is already known in the situation.

• el documento = the document
• un documento = a document

So:

• Preguntamos si el documento necesita una apostilla. = a particular document
• Preguntamos si un documento necesita una apostilla. = much more general, like asking about any document of that type

Because this is a normal indirect question about a factual matter: whether the document needs an apostille or not.

Spanish usually uses the indicative after verbs like preguntar, saber, averiguar, etc. when introducing an indirect question:

• Preguntamos si necesita...
• No sé si viene.
• Quiero saber si funciona.

So necesita is the natural choice here.

Necesita is the third-person singular present indicative of necesitar.

Here it agrees with el documento, which is singular:

• el documento necesita = the document needs

If the subject changed, the verb would change too:

• los documentos necesitan = the documents need

Because there is no direct object that is a specific person.

Also, el documento is not the object of preguntamos. It is the subject of the second clause:

• main clause: Preguntamos
• embedded clause: si el documento necesita una apostilla

You would use a if you said who you asked:

• Preguntamos al funcionario si el documento necesita una apostilla.
• We asked the official whether the document needs an apostille.

So:

• preguntar algo
• preguntar si...
• preguntar a alguien...

Because Spanish does not change the word order the same way English does in this kind of structure.

English often uses does:

• Does the document need an apostille?

But Spanish does not use an extra verb like do/does here. It keeps normal statement order:

• El documento necesita una apostilla.

Inside the indirect question, Spanish keeps that same normal order:

• Preguntamos si el documento necesita una apostilla.

So this is very natural Spanish syntax.

Because apostilla is a countable noun here, so Spanish normally uses an article with it.

• una apostilla = an apostille

Saying just necesita apostilla would sound incomplete in standard Spanish in this context.

Also, apostilla is a feminine noun, so it takes una:

• la apostilla
• una apostilla

In administrative and legal language, una apostilla usually refers to an official certification, especially the Hague Apostille, used to validate documents for use abroad.

So this is not just any note or annotation. In this context it is a specific legal certification attached to a document.

You can add the person after preguntar with a:

• Preguntamos al funcionario si el documento necesita una apostilla.
• We asked the official whether the document needs an apostille.

Or with an indirect object pronoun:

• Le preguntamos si el documento necesita una apostilla.
• We asked him/her whether the document needs an apostille.

So your original sentence leaves out who was asked, either because it is unknown, unimportant, or already understood.

A good approximate pronunciation is:

• a-pos-TEE-ya

A few notes:

• the stress is on -ti-: aposTILla
• the ll in most of Spain is pronounced like a y sound
• the a and o are clear Spanish vowels, not reduced like many English vowels

So a rough English approximation is ah-pos-TEE-yah.