Let $X$ be a smooth complex projective variety, $\dim(X)>3$ and $D$ be a very ample possibly reducible divisor on $X$. Is it true that $\textrm{Pic}(X)\cong \textrm{Pic}(D)$? If not, what would be a reasonable condition on $D$ (for example, would it suffice that $D$ is irreducible)? 


(if you have a precise reference, I would be especially grateful).



Picard group of a very ample divisor in a smooth variety of dimension >3
The answer is **yes** under very mild assumptions. In fact there is the following result.



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> **Proposition.** Let $L$ be a $k$-ample line bundle on a normal, irreducible, projective variety $X$ with at most Cohen-Macauley singularities. Assume that the dimension of the locus of non-rational singularities of $X$ is at most $0$ and that $D \in |L|$ is a divisor such that $X-D$ is a local complete intersection. Then, under the restriction mapping, we have $\textrm{Pic}(X) \cong \textrm{Pic}(D)$ if $\dim X \geq 4+k$ and $\textrm{Pic}(X) \to \textrm{Pic}(D)$ is injective with torsion free cokernel if $\dim X = 3+k$.
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The case you are interested in corresponds to $k=0$. For instance, if $X$ is smooth (of dimension at least $4$) then all the assumptions are fulfilled and what you want is true. 


The Proposition above is a consequence of Hamm's Lefschetz Theorem. For further details, see Beltrametti-Sommese, *The adjunction theory of complex projective varieties*, Corollary 2.3.4 page 51.


**Remark.** Given an integer $k \geq 0$, a line bundle $L$ on a projective variety $X$ is said to be $k$-ample if $a L$ is spanned for some $a > 0$, and the morphism $X \to \mathbb{P}^{h^0(aL)-1}$ defined by $|a L|$ has all fibers of dimension $\leq k$. For $k=0$, this is one of the basic characterization of ampleness.


