The main character gets some armor, which makes him basically invincible, for the purpose of killing dragons. Later on the he learns that he is actually a dragon with a powerful spell cast on him to keep him in a humanoid (elf I think) form. He breaks the spell and becomes a dragon, then gets some kind of dragon armor that makes him more powerful, to fight evil dragons. It may have been a trilogy, or even six books.
I believe this is Mystara: The Dragonlord Chronicles Book Series by Thorarinn Gunnarsson. This sounds like the plot of the first book in the series: Dragonlord of Mystara.
The main character is an orphan who gains the armor of the DragonLord. Due to their fear of a prophecy and the powers of the Dragonlord, dragons try to find and kill the main character. Eventually, we learn that he is actually a dragon himself and destined to be the dragonking.