Okular doesn't free memory at pages scrolling. Works on pdf and djvu files. Video describes the problem: https://youtu.be/E5QQ9hDXkyk
Created attachment 110669 [details] demonstration file
That's not a leak, it's just Okular using your available memory for caching. If you want it to use less memory even though there's plenty available, you can change it in Settings > Configure Okular > Performance. *** This bug has been marked as a duplicate of bug 388854 ***