Because "who" (or "quién) is always used with 3rd peron singular verbs, even in English. Don't you say "Who has a gun?", or Who gives a damn?", or "Who walks at night with no legs?"
"Quien tiene los documentos?" = Who has the documents? If it were third person plural, it would have to be "Quienes tienen los documentos?"
It could be plural if you knew the documents were held by a group! In this case it is provided to you.
Whispered agent 001 to agent 008, they both looked stressed while they hectically, but skilled ploughed through the office...
someone wants to continue;)?
008 saw a secretary with what looked like the documents in question, he walked/jogged up to her, asked for the papers, and tentitively looked at the top secret documents agents 001 and 008 had lost, only to find that it was a paper saying what people were going to bring for the upcoming potluck. The boss would not be happy, since the papers were plans for an agents suprise birthday party at the potluck
I got confused for a bit, too, before I realised Qúien is just a singular "Who?".