SUMMARY
The search bar currently appears at the bottom of the application and takes up the full horizontal distance.

This has multiple disadvantages:

1. In the case of a floating windows, the entire find bar can easily disappear outside the display area. Just move the window down a bit and invoke the Find interface. You will not be able to see the find bar at all.
2. Displaying the search bar at the very bottom requires the user to shift their center of attention from the top of the screen to very bottom (whereas most applications display their tools at the top, including the find button in Okular)
3. The horizontally maximized find bar obscures much more content than it needs to. It is very rare that someone would type more than about ten characters into the search interface, so the space is stolen from the content and wasted.
4. The prev/next result buttons in the horizontally maximized find bar are located on the opposite end of the screen from where the user enters the keyword (the location of the cursor). A compact search bar would ensure that these buttons are close to the cursor.

I think these are compelling reasons to change the UI. The current Find UI does not appear to offer any advantages - only disadvantages, and pretty serious ones.

One problem is consistency with other applications like Kate. Kate displays the find bar at the bottom, like many other text editors. However, Kate could probably also benefit from moving this interface to the top right (see Gnome builder https://d2.alternativeto.net/dist/s/gnome-builder_443674_full.png?format=jpg&width=1600&height=1600&mode=min&upscale=false)