[raaaainm]

PbF₂ (s) is partially soluble in water. Suppose there is a saturated solution is in equilibrium with the solid lead (II) fluoride. Why does the addition of nitric acid cause more solid lead (II) fluoride to dissolve?

There are no common ions to shift equilibrium, and the only thing I can imagine is that excess H+ allows for the preferable formation of HF.

[Corribus]

Could be part of it. What about the nitrate?

[raaaainm]

The nitrate could complex with Pb to form Pb(NO3)₂, but as nitrate adducts are soluble in water, it should remain as Pb²⁺ and NO₃^2-

[AWK]

There are no common ions to shift equilibrium, and the only thing I can imagine is that excess H+ allows for the preferable formation of HF.

and this is a good idea.