Translation:De geheime woorden verdwijnen nadat je ze hebt gelezen.
I don't understand the word order of "je ze hebt gelezen." why wouldn't it be "je hebt ze gelezen"
It's a subordinate clause, so all verbs go to the end.
Why can't we say "je hun hebt gelezen"?
I think it's because you only use hen or hun to refer to humans. In this sentence you are refering to "the secret words" which is not human, thats why you say ze