La novela tiene seiscientas páginas, pero mi amiga acabó leyéndola en una semana.

Because páginas is feminine plural, and the hundreds part of the number agrees with the noun in gender.

• seiscientos libros
• seiscientas páginas

Only the -cientos / -cientas part changes. The seis part stays the same.

Here acabó + gerund means ended up doing something. So acabó leyéndola means she ended up reading it or she wound up reading it.

This is slightly different from a plain statement like she finished reading it. It often suggests a final outcome, sometimes with a nuance of surprise or contrast.

No. They are different structures.

• acabó leyéndola = she ended up reading it
• acabó de leerla = usually she has just finished reading it / she had just finished reading it, depending on context

So this sentence is not using acabar de + infinitive.

The verb is leer. Its gerund is leyendo, not leendo.

This happens because some verbs with adjacent vowels insert y in certain forms:

• leer → leyendo
• oír → oyendo
• construir → construyendo

So leyendo is the correct form for reading.

Because la is the direct object pronoun meaning it, referring back to la novela.

In Spanish, object pronouns can be attached to a gerund:

• leyéndola = reading it

So the sentence literally has the idea my friend ended up reading it.

Yes. That is also possible.

With structures like this, the object pronoun can often go in either position:

• mi amiga acabó leyéndola
• mi amiga la acabó leyendo

Both are grammatical. The version in the sentence is very natural.

Because when the pronoun la is attached to leyendo, the written accent keeps the original stress in the right place.

• leyendo is stressed on yen
• when la is added, Spanish writes leyéndola to show that the stress stays there

Without the accent mark, the stress would be read differently.

Because the sentence presents this as a completed event: your friend finished the action within a defined time.

The preterite acabó fits a finished, whole event in the past.
The imperfect acababa would suggest background, repetition, or an unfinished viewpoint, which is not what this sentence is doing.

Here en expresses the time taken to complete something.

• acabó leyéndola en una semana = she ended up reading it in a week

This means one week was the total time needed to finish it.

In this sentence, it means the novel was completed over the course of one week — essentially in/within a week.

Because the sentence is about finishing a task, en una semana is understood as the amount of time it took, not a week from now.

Pero sets up a contrast:

• the novel is long: seiscientas páginas
• but your friend still finished it in only a week

So pero highlights that the second part is a bit surprising compared with the first.