Learn Russian: Я прочитал резюме кандидата, хотя оно очень длинное. - I read the candidate's resume, although it is very long.

Хотя means "although" or "even though." It connects two contrasting ideas: completing the reading versus the resume being very long. In Russian, you always put a comma before хотя when it introduces a new clause like this.

Why do we use the neuter pronoun оно in the second half of the sentence?

Оно refers back to резюме (resume). Even though резюме is a borrowed foreign word that doesn't change its endings (it is indeclinable), it ends in -е, so it is treated as a neuter noun. That is why it takes the neuter pronoun оно and the neuter adjective длинное.

Why is there an -а at the end of кандидата?

It is in the genitive case, which is used here to show possession, translating to "the resume of the candidate". Since кандидат is a masculine noun ending in a consonant, we add -а to form the genitive.

Why use the perfective verb прочитал instead of the imperfective читал?

Прочитал emphasizes the result: the action was fully completed. The speaker read the entire resume from start to finish. This makes the contrast with "although it is very long" much stronger than if they had just spent some time reading it (читал).