Of course you can always open up the PSPDFKit.bundle and replace files there, but it's not the recommended way, and it's hard to make sure you update this for each and every update we release.

The better way is to register a custom handler via imageLoadingHandler on the PSPDFKit object. This takes a block, gives you the image name and allows you to return a custom image. If you return nothing, the default PSPDFKit image will be used. It's best to set this block in the AppDelegate or before any UIViewController is on screen. Images are cached, so a late change will not do anything.