Is it true that the dual of a smooth hypersurface $X$ of $\mathbb{P}^n$ of degree $d\ge 2$ is a hypersurface? If yes, could you give me a simple proof ? Or a reference?


Note that in this case, the dual is birational to $X$.



Dual of a smooth hypersurface
Yes, the dual is a hypersurface. Here is a simple proof when $n\geq 4$. First of all, by the Lefschetz hyperplane theorem, the restriction homomorphism, $$ r : \text{Pic}(\mathbb{P}^n) \to \text{Pic}(X),$$ is an isomorphism. In particular, every invertible sheaf on $X$ is either ample, trivial, or anti-ample. 


Denote the Gauss map on $X$ by, $$ G : X \to (\mathbb{P}^n)^\vee, \ \ x \mapsto \mathbb{T}\_xX, $$ where $\mathbb{T}\_x X$ denotes the tangent hyperplane to $X$ at $x$.
The invertible sheaf $G^\*\mathcal{O}(1)$ is either ample, trivial or anti-ample. If $G$ is nonconstant, then $G^\*\mathcal{O}(1)$ has positive degree Hilbert polynomial, hence $G^\*\mathcal{O}(1)$ is neither trivial nor anti-ample. Therefore $G^\*\mathcal{O}(1)$ is ample. In particular, $G$ can have no positive dimensional fiber, since this would give a subvariety of $X$ on which $G^\*\mathcal{O}(1)$ is not ample. Therefore $G$ is finite, and hence $\text{dim}(G(X))$ equals $\text{dim}(X)$. 


Finally, if $G$ were constant with image $[H]$, for $H$ a fixed hyperplane in $\mathbb{P}^n$, then for every $x\in X$, the tangent hyperplane to $\mathbb{T}\_x X$ equals $H$. In particular, for every $x\in X$, $x$ is contained in $H$. Thus $X$ is contained in $H$, contradicting that $X$ is a smooth hypersurface of degree $d>1$.


**Edit.** I just want to add, I heard this argument long ago, but I do not know who first proved this result. Fyodor Zak has done important work on the Gauss map, so you might consult his work to try and find a reference.


**Second edit.** The OP asks about $n=3$. One could probably adapt the "fast" argument above using Noether-Lefschetz, but it is probably more honest just to compute $G^\*\mathcal{O}(1)$. Indeed, the morphism $G$ arises from the short exact sequence sequence,
$$ 0 \to \mathcal{O}\_{\mathbb{P}^n}(-d)|\_X \xrightarrow{u} \Omega\_{\mathbb{P}^n/k}|\_X \to \Omega\_{X/k} \to 0,$$
and the Euler sequence (restricted to $X$),
$$ 0 \to \Omega\_{\mathbb{P}^n/k}|\_X \xrightarrow{v} \mathcal{O}\_{\mathbb{P}^n}(-1)^{\oplus(n+1)}|\_X \to \mathcal{O}\_{\mathbb{P}^n}|\_X \to 0. $$
The adjoint of $v\circ u$ is an invertible quotient,
$$ \mathcal{O}\_{\mathbb{P}^n}(1)^{\oplus(n+1)}|\_X \to \mathcal{O}\_{\mathbb{P}^n}(d)|\_X, $$
which is equivalent to an invertible quotient (after twisting),
$$\mathcal{O}\_{\mathbb{P}^n}^{\oplus(n+1)}|\_X \to \mathcal{O}\_{\mathbb{P}^n}(d-1)|\_X. $$
This invertible quotient defines the morphism $G:X\to (\mathbb{P}^n)^\vee$. Thus, $G^\*\mathcal{O}(1)$ is isomorphic to $\mathcal{O}\_{\mathbb{P}^n}(d-1)|\_X$. Thus, if $d>1$, then $G^\*\mathcal{O}(1)$ is ample. Hence $G$ is finite.


