Learn Swedish: Chefen ber honom att granska sin dokumentation. - The boss asks him to review his own documentation.

In simple sentences, sin does refer back to the grammatical subject. However, this sentence contains an infinitive clause (att granska...). In these structures, sin refers to the logical subject—the person actually performing the action of that specific verb. Since honom ("him") is the one doing the reviewing, sin points back to him. It is his own documentation!

How would I change this sentence to say he is reviewing the boss's documentation?

You would replace the reflexive sin with a regular possessive pronoun like hans (his) or hennes (hers). The sentence would become Chefen ber honom att granska hans/hennes dokumentation. Because these pronouns aren't reflexive, they look outside the infinitive clause to find their owner (the boss).

Why does dokumentation take sin instead of sitt?

The choice between sin, sitt, and sina always depends on the noun being owned, not the owner. Because dokumentation is a singular en-word (en dokumentation), it must take sin.

Does the verb ber always require att before the next verb?