Pentax K-S2 has a 20.0MP APS-C
(23.5 x 15.6 mm ) sized CMOS sensor and features PRIME MII processor. On the other hand, Nikon D3400 has a 24.0MP APS-C
(23.5 x 15.6 mm ) sized CMOS sensor and features Expeed 4 processor.
Another similarity between these two cameras is that both Nikon D3400 and Pentax K-S2 sensors lack anti-alias (Low-Pass) filters. Removing anti-alias filter increases the sharpness and level of detail but at the same time, it increases the chance of moiré occurring in certain scenes.
Below you can see the K-S2 and D3400 sensor size comparison.
Pentax K-S2 and Nikon D3400 have the same sensor sizes so they will provide same level of control over the depth of field when used with same focal length and aperture. On the other hand, since Pentax K-S2 has 20% larger pixel area compared to Nikon D3400, it has potential to collect more light on pixel level hence have less noise in low light / High ISO images.