The PC-24 has, under the fuselage, a kind of huge support to hold the wings.
If we compare with the Citation CJ4 or the Phenom 300, this difference is quite significant. Why??
To add to the other answer...
The Pilatus PC-24 is billed as a business jet that can operate out of rough airfields. Comparing the pictures of the PC-24 to the Phenom 300 or CJ4, you can see that there is quite a bit of extra ground clearance. You can also see that the rear landing gear on the PC-24 are much more robust (dual wheels, longer travel length) than the other two aircraft.
If you look at this picture:
You can see that the wheels also retract into that area. They probably wanted to keep the wing low to take advantage of ground-effect to improve short-field performance. So having a higher-up fuselage allowed them to put the engines up higher and avoid FOD damage, while the low wing allowed them an impressive short field take-off performance. The PC-24 can operate out of a field just 2,800 feet long. The Phenom 300 takes 3,200 feet and the CJ4 takes about 3,500.
So the "bump" is there to be able to handle additional loads, elevate the fuselage, and store the gear. I can't find any diagrams of the fuel system, but it's possible that it also houses fuel tanks or baggage.
The huge support, or bulge, is a fairing, designed for reduction of wing root drag. So many people talk about wing tip vortex drag, but much more is created at the wing/fuselage interface, especially in uncoordinated flight. This improvement, first seen in 1930s vintage gliders (where else!), improves the airflow around the aircraft, resulting in significant fuel saving.
Importantly, this is a subsonic effect. Many super sonic aircraft actually reduce fuselage diameter near the wing to reduce drag, complying with "Area Rule".
In that area, the wing spar is interfacing with the fuselage, and the retractable landing gear needs space.
In larger airliners, that space is largely inside the cylindrical fuselage, in the level underneath the passenger floor, and the cabin is still high enough to walk in the aisle.
The cross section of the business jets is usually quite small, so having the spar crossing the cabin would make a bump in the cabin floor. In order to not reduce cabin height, that structural space gets pushed "slightly" outside of the cylindrical fuselage body.
That said, other large airliners also have a large bulge - check the A380 for example.
I did some more digging into that.. Let's focus on a comparison of the PC24 and the CJ4.
I didn't find any reliable drawings or models, so while the bulge certainly seems bigger in the pictures, it's hard to tell how much bigger it really is. Also - designs are always a compromise, and we do not know all the boundary conditions Pilatus or Cessna had, so we can only make educated guesses as to why the bulge is larger on one than on the other.
In order to explain different designs, best look at the differences in mission profile. A big difference between the CJ4 and the PC24 I know of is that the PC24 has "rough field" certification. And here we can start to dig..
The CJ4 has a "single wheel" main landing gear (one left, one right), which retract into the wing, so it doesn't need any space in the bulgy fairing. The PC24 has twin wheels, and they do retract into that bulgy fairing, likely because they are too wide to fit into the wing. The wing also seems to have a more elaborate flap system, likely leaving even less space there for a retracting landing gear. I would attribute both to the rough field certification:
Concerning the aerodynamics: the bulge doesn't strike me as particularly aerodynamic, and if it was, it shouldn't be a differentiating factor between the PC24 and the CJ4. Cruise speeds between the two are similar (Wikipedia: 440kn for the PC24 vs 451kn for the CJ4), assuming that cruise aerodynamic efficiencies are also similar, both planes should converge there in the design. Assuming similar efficiencies there is valid because good aerodynamic knowledge for cruise operating points is around for longer than both planes exist.
So .. Where do they differ? The PC24 as the younger plane might have profited from newer fluid dynamics methods, CFD, turbulence & separation prediction. If anything - these improvements won't be so visible during cruise, but maybe improve handling at slow speeds, stall characteristics etc. It would strike me as odd though, if the bulge improves stall characteristics.
Bottom line for aerodynamics: I strongly doubt that the bulge is larger in the PC24 for aerodynamic reasons.
Finally, another example to support my point: the Hawker 800 has also twin wheels on the main landing gear, and it also has a relatively large bulge:
Note: all pictures are from Wikipedia.