  We study the effects of dark energy (DE) anisotropic stress on features of
the matter power spectrum (PS). We employ the Parametrized Post-Friedmannian
(PPF) formalism to emulate an effective DE, and model its anisotropic stress
properties through a two-parameter equation that governs its overall amplitude
($g_0$) and transition scale ($c_g$). For the background cosmology, we consider
different equations of state to model DE including a constant $w_0$ parameter,
and models that provide thawing (CPL) and freezing (nCPL) behaviors. We first
constrain these parameters by using the Pantheon, BAO, $H_0$ and CMB Planck
data. Then, we analyze the role played by these parameters in the linear PS. In
order for the anisotropic stress not to provoke deviations larger than $10\%$
and $5\%$ with respect to the $\Lambda$CDM PS at $k \sim 0.01 \,h/\text{Mpc}$,
the parameters have to be in the range $-0.30< g_0 < 0.32$, $0 \leq c_g^2 <
0.01$ and $-0.15 < g_0 < 0.16$, $0 \leq c_g^2 < 0.01$, respectively.
Additionally, we compute the leading nonlinear corrections to the PS using
standard perturbation theory in real and redshift space, showing that the
differences with respect to the $\Lambda$CDM are enhanced, especially for the
quadrupole and hexadecapole RSD multipoles.
