How much pressure does the compressor of an engine of B787 exert on air?
Wikipedia will tell you that two engines are offered for the 787, namely the General Electric GEnx and the Rolls-Royce Trent 1000. Their overall compression ratios are given as 52:1 for the Trent 1000 and 41:1 for the GEnx.

Note, however, that this just one point which will change over the whole operating envelope. In case of the Trent 1000, the point is given as "top of climb", whereas the number for the GEnx is given without context. At high speed, the intake will already raise the pressure at the compressor entry by approximately 20%.

If we now assume that ambient pressure at the (not specified) top-of-climb altitude is a quarter of sea level pressure and add the intake pressure increase, the total pressure of the air inside the engine at the compressor exit of the Trent 1000 is 1,580,000 Pascal or 15.6 times sea level pressure.

In the static case (at rest, on the ground), the engine needs to actively suck in air, so the pressure at the compressor entry is lower than ambient (approximately 88% of ambient pressure). If we now assume that the compression ratio is around 45, the absolute pressure at the compressor exit would be 4,012,500 Pascal or 40 times sea level pressure.