The name "Hadi" (ہادی) comes from the root word for guidance. It carries two meanings: (1) Isal il al-Matlub: leading someone to the desired goal, and (2) Ira'at al-Tariq: showing the way.
The first meaning is exclusive to Allah the Almighty, as only He grants the ability to reach the true goal of guidance. The second meaning, however, is general, since any person can show the way to others. In fact, "Hadi" is one of the names of the Prophet ﷺ.
Therefore, based on the second meaning, it is permissible to use the name "Hadi" without adding the prefix Abd (servant of).
Allah the Almighty knows best.
Darul Ifta Al-Ikhlas, Karachi.