Afrikaans spelling is often more phonetic than English spelling. So many words that came from English, Dutch, or other languages are spelled in a way that matches Afrikaans pronunciation more closely.
That is why English document becomes Afrikaans dokument.
A rough guide for English speakers is:
dee res iss in dee doh-kuh-ment
A few notes:
Pronunciation can vary a little by speaker and accent, but dee res is in dee dokument is a good starting point.
In standard Afrikaans, die most commonly means the.
Depending on context, it can also be used in ways that point something out, especially in relative structures or certain emphatic uses, but in a basic sentence like Die res is in die dokument, it simply means the.
So for a learner, the correct reading here is definitely the rest is in the document.