Это официальное письмо, поэтому лучше не использовать сокращения.

Because the adjective has to agree with the noun in gender, number, and case.

• письмо is neuter
• it is singular
• here it is in the nominative case

So the adjective must also be neuter singular nominative:

• официальное письмо

That is why the ending is -ое.

Here это means something like this is or it is in an identifying sentence.

So Это официальное письмо is a very natural way to say that the thing being discussed is an official letter.

Compare:

• Это официальное письмо = This/It is an official letter
• Письмо официальное = The letter is official

Both are possible, but they have a slightly different feel.
Это официальное письмо sounds like you are identifying what kind of thing it is.
Письмо официальное sounds more like you are stating a property of the letter.

Russian does not have articles like a, an, and the.

So официальное письмо can mean:

• an official letter
• the official letter
• sometimes just official letter

The exact meaning depends on context. In this sentence, English would usually use an or the, but Russian simply does not need an article.

Because the sentence has two parts:

• Это официальное письмо
• поэтому лучше не использовать сокращения

The second part gives the result/consequence of the first.
поэтому means therefore / so / that’s why.

In Russian, when two clauses like this are joined, a comma is normally used:

• Это официальное письмо, поэтому ...

You could also split it into two sentences, but the comma version is very standard.

Поэтому is a linking word meaning therefore, so, or for that reason.

It connects the first idea to the consequence:

• it is an official letter
• therefore it is better not to use abbreviations

English speakers sometimes confuse поэтому with потому что:

• поэтому = therefore / so
• потому что = because

So:

• Это официальное письмо, поэтому... = This is an official letter, so...
• ...потому что это официальное письмо = ...because this is an official letter

Because this is an impersonal recommendation.

Russian often leaves out a specific subject when the meaning is general, like English it is better not to... or you shouldn’t...

So лучше не использовать сокращения means something like:

• it is better not to use abbreviations
• you’d better not use abbreviations
• one should preferably not use abbreviations

The sentence is aimed at the reader in a general way, but Russian does not need to say вы here.

Yes. Лучше is the comparative form of хорошо and means better.

But in Russian, лучше + infinitive is a very common way to give advice or make a recommendation:

• лучше подождать = it’s better to wait
• лучше спросить = it’s better to ask
• лучше не использовать = it’s better not to use

So even though it literally looks like better, the whole structure functions like practical advice.

Because after лучше in this kind of recommendation, Russian usually uses the infinitive.

The infinitive names the action in a general way:

• лучше не использовать сокращения

This is like saying better not to use abbreviations.

Russian often uses infinitives in impersonal advice structures:

• нужно сделать = it is necessary to do
• можно войти = it is possible to enter / you may enter
• лучше не спешить = it is better not to hurry

Because не normally goes directly before the verb it negates.

Here the action being negated is using:

• не использовать = not to use

So:

• лучше не использовать сокращения = it is better not to use abbreviations

This is the standard position. Putting не somewhere else would either sound wrong or change the meaning.

Here сокращения is in the accusative plural because it is the direct object of использовать.

You are using what?
→ сокращения

The singular noun is:

• сокращение = abbreviation / shortened form

Its plural is:

• сокращения

For inanimate plural nouns in Russian, the accusative plural is the same as the nominative plural. That is why it looks just like the basic plural form.

So:

• nominative plural: сокращения
• accusative plural: сокращения

Not necessarily. Сокращение can refer to different kinds of shortened forms, depending on context:

• abbreviations
• shortened words
• contractions
• sometimes more generally short forms

In this sentence, because we are talking about an official letter, сокращения most naturally means abbreviations or informal shortened forms that are better avoided in formal writing.

Yes. Russian has several natural ways to express this idea, but the tone changes a little.

• лучше не использовать сокращения
  = polite advice, fairly neutral

• не надо использовать сокращения
  = there’s no need / you shouldn’t use abbreviations, a bit more direct

• не стоит использовать сокращения
  = it’s not advisable to use abbreviations, also very natural

• не следует использовать сокращения
  = more formal, often used in writing or instructions

So the original version is a good neutral choice for giving advice.

Yes, Russian word order is flexible, although the original is the most neutral.

For example, you could also say:

• Это официальное письмо, поэтому сокращения лучше не использовать.
• Это официальное письмо, поэтому не стоит использовать сокращения.

The original order:

• поэтому лучше не использовать сокращения

sounds smooth and standard. Changing the order usually changes emphasis, not the basic meaning.