PRATISMRITI

Yudhishtira was instructed Pratismriti by Veda Vyasa ,to be passed on to Arjuna who was in need of it not Yudhishtir.

That foremost of speakers then, the son of Parasara, taking Yudhishthira to a corner, began to address him in words of deep import, saying, 'O best of the Bharatas, the time is come for thy prosperity, when, indeed Dhananjaya--that son of Pritha--will slay all thy foes in battle. Uttered by me and like unto success personified, accept from me this knowledge called Pratismriti that I impart to thee, knowing thou art capable of receiving it. Receiving it (from thee), Arjuna will be able to accomplish his desire. And let Arjuna, O son of Pandu, go unto Mahendra and Rudra, and Varuna, and Kuvera, and Yama, for receiving weapon from them. He is competent to behold the gods for his asceticism and prowess. He is even a Rishi of great energy, the friend of Narayana; ancient, eternal a god himself, invincible, ever successful, and knowing no deterioration. Of mighty arms, he will achieve mighty deeds, having obtained weapons from Indra, and Rudra, and the Lokapalas,.'" 

1. Yudhishtira is capable to receive it from him and instruct it to Arjuna.
2. With the help of the mantra Arjuna will accomplish his purpose
3. Arjuna is ancient Rishi Nara,the companion of Narayana himself.
4. With his penance and prowess Arjuna will behold the gods and get astras from them.
5. Arjuna will then slay all the foes of Yudhishtira and bring prosperity.

Veda Vyasa then instructed the mantra to Yudhishtira, and disappeared.

Yudhishtira retained the mantra in his mind and recited it on proper occasions. Obviously he has the task of passing it on to Arjun ..in the meanwhile they changed their abode to Kamyaka forest.