Pagdating ni Pedro sa opisina, pumirma siya sa dokumento.

Pagdating is a noun-like form built from dating/dating-related verbal material meaning arrival or coming. In this sentence, Pagdating ni Pedro sa opisina means upon Pedro’s arrival at the office or more naturally when Pedro arrived at the office.

So although it looks like one word, it functions as part of a time expression.

Because in pagdating ni Pedro, the word Pedro is not the main topic of a full clause. It is the doer/owner of the action inside a noun-like expression.

• si Pedro is used when Pedro is the focused personal name in a full clause.
• ni Pedro is the genitive form, often used for of Pedro, by Pedro, or the actor inside a nominalized expression.

So:

• dumating si Pedro = Pedro arrived
• pagdating ni Pedro = Pedro’s arrival / when Pedro arrived

Sa is the general marker for location, direction, or destination. Here, sa opisina means at the office or to the office, depending on context.

In this sentence, it tells you where the arrival happened:

• pagdating ni Pedro sa opisina = when Pedro arrived at the office

Not really. It is best understood as an introductory time phrase, not a complete standalone clause in this context.

It sets the time for the main action:

• Pagdating ni Pedro sa opisina = When Pedro arrived at the office
• pumirma siya sa dokumento = he signed the document

Together they form the complete sentence.

The comma separates the introductory time expression from the main clause.

So the structure is:

• Pagdating ni Pedro sa opisina, = introductory time phrase
• pumirma siya sa dokumento. = main clause

This is similar to English:

• When Pedro arrived at the office, he signed the document.

Pumirma comes from the root pirma (signature / sign) with the infix -um-, which forms an actor-focus verb.

Here, pumirma means signed or did the signing.

More specifically:

• root: pirma
• actor-focus verb: pumirma
• aspect: completed

So pumirma siya means he signed.

It is better to say it shows completed aspect, not tense in the same way English does.

Filipino verbs usually emphasize whether an action is:

• completed
• ongoing
• contemplated/not yet started

Here, pumirma shows the action is completed. In this sentence, the natural English translation is past:

• he signed

But the key grammatical idea in Filipino is completed action, not simply past tense.

Filipino commonly places the predicate first. In a verbal sentence, the verb often comes before the subject/topic.

So:

• pumirma siya = literally something like signed he
• natural English = he signed

This word order is normal in Filipino. Pronouns such as siya often come right after the verb.

With pumirma, the thing being signed is often introduced with sa in this kind of actor-focus construction.

So:

• pumirma siya sa dokumento = he signed the document

Grammatically, pumirma focuses on the actor, and sa dokumento is treated as a non-focus complement.

If you wanted to focus the document more directly, Filipino could use a different verb form. But in this sentence, pumirma sa dokumento is completely natural.

Yes, but the structure would change.

For example:

• Nang dumating si Pedro sa opisina, pumirma siya sa dokumento.

That also means When Pedro arrived at the office, he signed the document.

The original sentence uses a more compact nominalized expression:

• Pagdating ni Pedro sa opisina...

Both are natural, but pagdating is a very common way to form upon arrival / when someone arrived expressions.

Yes. You could move the time phrase later, for example:

• Pumirma siya sa dokumento pagdating ni Pedro sa opisina.

But the original order is often clearer and more natural when the speaker wants to establish the time first:

• Pagdating ni Pedro sa opisina, pumirma siya sa dokumento.

That is very similar to English starting with When Pedro arrived at the office...

In this sentence, ni Pedro belongs only to pagdating. It tells us that Pedro is the one who arrived.

Then the second clause says:

• pumirma siya sa dokumento = he signed the document

Normally, siya would be understood as referring to Pedro, because he is the person just mentioned. So the most natural reading is:

• Pedro arrived at the office.
• Pedro signed the document.

But grammatically, ni Pedro is tied specifically to arrival, while siya is the pronoun in the main clause.

Often yes, but not always. Pagdating sa... can also mean when it comes to... in other contexts.

For example:

• Pagdating sa trabaho, seryoso siya. = When it comes to work, he is serious.

But in your sentence, because it is followed by ni Pedro sa opisina, the meaning is clearly literal:

• Pedro’s arrival at the office / when Pedro arrived at the office