Eu leio a documentação necessária no site.

Yes. In Brazilian Portuguese, subject pronouns are often optional because the verb form already shows who is doing the action.

• Eu leio = I read
• Leio also naturally means I read

Including Eu can:

• add emphasis
• create contrast, as in Eu leio, mas ele não
• make the subject extra clear

So both are correct:

• Eu leio a documentação necessária no site
• Leio a documentação necessária no site

Leio is the present indicative form of the verb ler (to read) for eu.

Conjugation of ler in the present:

• eu leio
• você/ele/ela lê
• nós lemos
• vocês/eles/elas leem

In this sentence, leio usually means I read or I am reading, depending on context. Portuguese often uses the simple present where English might use either the simple present or present progressive.

In Brazilian Portuguese, leio is pronounced roughly like LAY-oh, but smoothly as one word.

A useful approximation:

• lei sounds like the English word lay
• then add o

So: LEI-o

The stress is on the first part:

• LEI-o

The a is the definite article the.

• a documentação = the documentation

Portuguese usually uses articles more often than English does. So where English might say I read required documentation, Portuguese often prefers I read the required documentation:

• leio a documentação necessária

Also, documentação is a feminine singular noun, so the article is a.

Because it agrees with documentação, which is a feminine singular noun.

In Portuguese, adjectives usually agree in gender and number with the noun they describe.

• documentação = feminine singular
• so the adjective must also be feminine singular: necessária

Compare:

• o documento necessário = the necessary document
• a documentação necessária = the necessary documentation
• os documentos necessários = the necessary documents
• as informações necessárias = the necessary pieces of information

In Portuguese, adjectives often come after the noun.

So:

• documentação necessária = necessary documentation

That is the most normal order here.

Sometimes adjectives can come before the noun, but that often sounds more literary, emphatic, or can slightly change the nuance. In this case, a documentação necessária is the standard, natural phrasing.

No site means on the website or on the site, depending on context.

It is made from:

• em = in/on/at
• o site = the site

These combine into:

• em + o = no

So:

• no site = on the site / on the website

In modern Brazilian Portuguese, site commonly means website.

Because Portuguese normally contracts certain prepositions with articles.

Here:

• em + o = no

Other common contractions:

• em + a = na
• em + os = nos
• em + as = nas
• de + o = do
• de + a = da

So em o site is not the normal form. The correct form is no site.

Yes. In Brazilian Portuguese, site very commonly means website.

So:

• no site usually means on the website

If the context is clearly digital, Brazilian speakers will naturally understand site as a website. You do not need a separate word like website in most everyday situations.

Yes, often it can.

Portuguese frequently uses the simple present where English might choose either:

• I read
• I am reading

So Eu leio a documentação necessária no site can mean:

• I read the required documentation on the site
• I am reading the required documentation on the site

If Brazilian Portuguese wants to emphasize an ongoing action right now, it can also use:

• Estou lendo a documentação necessária no site = I am reading the required documentation on the site

A little, but the original order is the most neutral and natural.

Standard order:

• Eu leio a documentação necessária no site.

Possible variations:

• No site, eu leio a documentação necessária.
  This puts more focus on on the site.
• A documentação necessária, eu leio no site.
  This is more marked and less neutral.

For a learner, the safest default is:

• subject + verb + object + location

So the original sentence is a very good model.

Yes, but the meaning changes a little.

• no site = on the website / on the site
• na página = on the page
• na internet = on the internet

So they are not always interchangeable.

Examples:

• Leio a documentação necessária no site.
  I read it on the website.
• Leio a documentação necessária na página de suporte.
  I read it on the support page.
• Leio a documentação necessária na internet.
  I read it on the internet.
  This is broader and less specific.

It is usually like documentation in English: an uncountable or collective noun.

• documentação refers to documentation as a whole
• documentos refers to individual documents

So:

• a documentação necessária = the necessary documentation
• os documentos necessários = the necessary documents

If you mean a collection of information, manuals, requirements, or official paperwork in general, documentação is the right choice. If you mean separate files or papers, documentos may be better.