Solving PDEs by a first integral of a vector field

Question

Consider a vector field on $\mathbb{R}^2$: $$X=a(u,v)\frac{\partial}{\partial u}+b(u,v)\frac{\partial}{\partial v}.$$ Given smooth functions $p(u,v), q(u,v)$, how do we solve $$Xf+pf=q$$ for $f(u,v)$ from a geometric viewpoint? I found something in the internet. If $p$ and $q$ are identically zero, then the job is done by examining a first integral of $X$. However, this is not the case. Does anyone have an idea? Thanks a lot.