  The recent realisations of hydrogen doped $Ln$FeAsO ($Ln$=Nd and Sm)
superconducting epitaxial thin films call for further investigation of their
structural and electrical transport properties. Here, we report on the
microstructure of a NdFeAs(O,H) epitaxial thin film and its temperature, field,
and orientation dependencies of the resistivity and the critical current
density $J_{\rm c}$. The superconducting transition temperature $T_{\rm c}$ is
comparable to NdFeAs(O,F). Transmission electron microscopy investigation
supported that hydrogen is homogenously substituted for oxygen. A high
self-field $J_{\rm c}$ of over 10 MA/cm$^2$ was recorded at 5 K, which is
likely to be caused by a short London penetration depth. The anisotropic
Ginzburg-Landau scaling for the angle dependence of $J_{\rm c}$ yielded
temperature-dependent scaling parameters $\gamma_{\rm J}$ that decreased from
1.6 at 30 K to 1.3 at 5 K. This is opposite to the behaviour of NdFeAs(O,F).
Additionally, $\gamma_{\rm J}$ of NdFeAs(O,H) is smaller than that of
NdFeAs(O,F). Our results indicate that heavily electron doping by means of
hydrogen substitution for oxygen in $Ln$FeAsO is highly beneficial for
achieving high $J_{\rm c}$ with low anisotropy without compromising $T_{\rm
c}$, which is favourable for high-field magnet applications.
