  In the fall of 2019, during the in-flight calibration phase of the SRG
observatory, the onboard eROSITA and Mikhail Pavlinsky ART-XC telescopes
carried out a series of observations of PG 1634+706 - one of the most luminous
(an X-ray luminosity $\sim 10^{46}$ erg/s) quasars in the Universe at $z<2$.
Approximately at the same dates this quasar was also observed by the XMM-Newton
observatory. Although the object had already been repeatedly studied in X-rays
previously, its new observations allowed its energy spectrum to be measured
more accurately in the wide range $1-30$ keV (in the quasar rest frame). Its
spectrum can be described by a two-component model that consists of a power-law
continuum with a slope $\Gamma\approx 1.9$ and a broadened iron emission line
at an energy of about 6.4 keV. The X-ray variability of the quasar was also
investigated. On time scales of the order of several hours (here and below, in
the source rest frame) the X-ray luminosity does not exhibit a statistically
significant variability. However, it changed noticeably from observation to
observation in the fall of 2019, having increased approximately by a factor of
1.5 in 25 days. A comparison of the new SRG and XMM-Newton measurements with
the previous measurements of other X-ray observatories has shown that in the
entire 17-year history of observations of the quasar PG 1634+706 its X-ray
luminosity has varied by no more than a factor of 2.5, while the variations on
time scales of several weeks and several years are comparable in amplitude.
