Understanding my combi-boiler
The boiler should not significantly affect your water pressure. Tankless boilers do put water through a somewhat more twisty path than a traditional water heater, but if the unit is properly sized for your installation and properly piped, the impact of this on water pressure should not be noticeable. If your hot water pressure, ideally tested from multiple faucets within your home, really is lower than your cold water pressure, your system may be incorrectly installed or require maintenance.

If your boiler has a pressure reducing valve, it is most likely the valve that regulates the pressure of cold water supply into your hydronic heating system. A picture and/or model number would help confirm.

Though you haven't said it explicitly, I'm guessing you are getting less water pressure from your shower than you'd like. If you aren't seeing lower hot water pressure at other fixtures, then it's also possible you have an issue with the mixing valve in your shower faucet. These can clog due to mineral buildup or perform incorrectly due to wear. You may also have a shower head that doesn't let much water through.