There are too many things that indicates Arjuna is the avtara of shiva.
Krishna and Arjun are friends , Vishnu and Shiva are also friends.
Arjun marries Subhadra , sister of Krishna . Parvati (wife of Shiva) is sister of Vishnu.(Biggest similarity) . Also Subhadra is probably avatar of Durga. Is this is correct , then Arjun is definitely Shiva as only Shiva can marry Durga.
Shiva is called PinakPani , i.e. someone who holds bow named Pinak . Pinak is the first bow. Arjun is the best Dhanurdhari.