Preciso de digitalizar o documento antes de o enviar.

In European Portuguese, precisar usually takes de when it means to need.

So in Portugal, preciso de digitalizar is the normal pattern.

• Preciso de dinheiro = I need money
• Preciso de sair = I need to leave
• Preciso de digitalizar o documento = I need to scan the document

A learner may also come across preciso digitalizar, especially in Brazilian Portuguese, but for Portuguese from Portugal, preciso de is the safer and more natural choice.

Because Portuguese often leaves out the subject pronoun when the verb already makes the subject clear.

Preciso already tells you the subject is I.

So:

• Preciso de digitalizar o documento = perfectly normal
• Eu preciso de digitalizar o documento = also correct, but more emphatic

You would add eu if you want contrast or emphasis, for example:

• Eu preciso de digitalizar o documento, mas tu não.

Yes, literally it is to digitize, but with o documento it very often means to scan the document or convert it into digital form.

In Portugal, digitalizar um documento is a very normal way to say scan a document.

Because antes de is the normal structure for before when it is followed by a noun or an infinitive.

Examples:

• antes de sair = before leaving
• antes do almoço = before lunch
• antes de o enviar = before sending it

On its own, antes usually works as an adverb meaning before/earlier:

• Cheguei antes. = I arrived earlier.

So in your sentence, antes de introduces the action that happens later.

That o is a direct object pronoun meaning it.

It refers back to o documento.

So:

• o documento: here o is the definite article, meaning the
• de o enviar: here o is a pronoun, meaning it

Because documento is masculine singular, the pronoun is o.

If the noun were feminine singular, you would use a instead.

Because o here is a pronoun, not an article.

Portuguese contracts de + o into do when o is an article:

• o livro = the book
• a capa do livro = the cover of the book

But in de o enviar, o means it, so standard Portuguese keeps them separate:

• antes de o enviar

So this is not the same kind of o as in do documento.

Yes, you may also see antes de enviá-lo.

With infinitives in Portuguese, object pronouns can often appear in different positions depending on the structure and style. In European Portuguese, antes de o enviar is very natural and common.

So these are both possible:

• antes de o enviar
• antes de enviá-lo

The sentence you were given uses a very common European Portuguese pattern.

After antes de, Portuguese normally uses the infinitive when the subject is understood from context.

Here, the person who needs to scan the document is also the person who will send it, so the infinitive works naturally:

• Preciso de digitalizar o documento antes de o enviar.

If you want to make the subject explicit, Portuguese can do that too:

• antes de eu o enviar = before I send it
• antes de ele o enviar = before he sends it

So the infinitive is used because the subject is already clear.

Yes, if the context already makes it clear what is being sent.

• Preciso de digitalizar o documento antes de enviar.

That can work, especially in conversation, because listeners will usually understand that the document is what is being sent.

But antes de o enviar is more explicit, because it clearly says before sending it. The pronoun removes any doubt about what the object is.