This is the 4656th most frequent Russian word.
документу
"Документу" is the dative case form of "документ," meaning "to the document" in English.
In this sentence, "документу" is in the dative case, indicating the recipient of the action (access is granted to the document).
Я получил доступ к важному документу.
I gained access to an important document.
Here, "документу" uses the dative case to show the target of an action—changes are directed towards the document.
Мы должны отправить изменения этому документу.
We need to send the changes to this document.
This usage is incorrect grammatically; the sentence should use "документе" (prepositional case for "in this document"). The example invites discussion about common mistakes and proper case usage.