O site ficou lento, então eu baixei o anexo e continuei o trabalho offline.

Because ficou lento means became slow or got slow, which suggests a change.

• ficou lento = it got slow / it became slow
• estava lento = it was slow

In this sentence, the speaker is describing what happened first: the site changed state, then they downloaded the attachment, then they kept working offline. So ficou is a very natural choice.

Here, ficou is the past form of ficar, and it means became or got.

Portuguese often uses ficar + adjective to express a change of condition:

• ficou lento = became slow
• ficou cansado = got tired
• ficou pronto = became ready / ended up ready

So O site ficou lento is literally something like The site got slow.

Because the sentence tells a sequence of completed actions in the past.

• ficou lento = the site became slow
• baixei o anexo = I downloaded the attachment
• continuei o trabalho offline = I continued the work offline

This is a very common use of the pretérito perfeito in Portuguese: it moves the story forward step by step.

That is true: Portuguese often omits subject pronouns when the verb form already makes the subject clear.

So both are natural:

• então eu baixei o anexo
• então baixei o anexo

Including eu can:

• add a little emphasis
• make the sentence clearer
• sound slightly more explicit in spoken or written narration

So eu is optional here, not required.

Yes, baixar can mean to lower, but in computer and internet contexts it very commonly means to download in Brazilian Portuguese.

So:

• baixar um arquivo = to download a file
• baixei o anexo = I downloaded the attachment

This is one of the most common tech uses of baixar in Brazil.

Here, anexo means attachment, usually an email attachment or a file attached to a message/system.

So:

• o anexo = the attachment
• baixei o anexo = I downloaded the attachment

In other contexts, anexo can also mean something like annex or attached, but in this sentence the file-related meaning is the natural one.

Portuguese often uses definite articles more than English does.

• continuei o trabalho = I continued the work
• In natural English, we might simply say I continued working or I kept working

Using o makes the work feel specific: the work I was already doing.

Portuguese frequently says:

• fechei a porta
• lavei as mãos
• continuei o trabalho

where English may or may not use the.

Yes, absolutely. Both are natural, but they are structured differently.

• continuei o trabalho offline = I continued the work offline
• continuei trabalhando offline = I kept working offline

The version in your sentence sounds a bit more noun-based and concrete, focusing on the work itself. The version with trabalhando focuses more on the ongoing action of working.

Both are good Portuguese.

Yes, it is very normal, especially in technology or work-related contexts.

Brazilians often use English tech words such as:

• offline
• online
• download
• link
• login

So continuei o trabalho offline sounds natural. Depending on context, you could also hear more descriptive alternatives such as:

• sem internet
• sem conexão
• fora da internet

But offline is very common and idiomatic.

Here, então means so.

The logic is:

• the site became slow
• so I downloaded the attachment
• and continued working offline

In other contexts, então can also mean then, depending on how it is used. So it is a flexible connector.

Examples:

• Estava chovendo, então fiquei em casa. = It was raining, so I stayed home.
• Primeiro fiz isso, então fiz aquilo. = First I did this, then I did that.

Because it agrees with site, which is treated as a masculine noun in Portuguese: o site.

So:

• o site ficou lento
• a página ficou lenta

The adjective must match the noun in gender and number.

It can be rearranged somewhat, though the original version is very natural.

Possible alternatives:

• O site ficou lento, então baixei o anexo e continuei o trabalho offline.
• Então eu baixei o anexo e continuei o trabalho offline porque o site ficou lento.
• Como o site ficou lento, eu baixei o anexo e continuei o trabalho offline.

The original sentence is straightforward and conversational: cause first, then consequence.

It sounds very natural.

It uses several things that are common in everyday Brazilian Portuguese:

• ficar + adjective for a change of state
• baixar meaning download
• então as a connector meaning so
• offline in a tech/work context

So this is the kind of sentence a Brazilian Portuguese speaker could easily say or write in a modern work setting.