7. In the quadrilateral pyramid $P-ABCD$, plane $PAD \perp$ plane $ABCD$, $\triangle PAD$ is an equilateral triangle with side length 2, and the base $ABCD$ is a rhombus with $\angle BAD=60^{\circ}$. Then the size of the acute dihedral angle formed by plane $PAD$ and plane $PBC$ is $\qquad$