Let $G$ be a discrete finitely generated group which acts properly and freely on a smooth manifold $M$ with compact quotient $M/G$. Is it right to consider any function $f \in C^{\infty}_c(M)$ (with compact support) as smooth section of some finite vector bundle $f' \in C^{\infty}(M/G, E)$ and then as smooth section of corresponding $G$-equivariant bundle $ C^{\infty}_c(M, \hat{E})$?
If it is correct can I define a Fredholm differential operator $D : C^{\infty}_c(M) \rightarrow C^{\infty}_c(M)$ as $G$-invariant operator on $C^{\infty}_c(M, \hat{E})$?