Often I want to read a PDF while cooking dinner. The projector is visible from the stove, but the desk is to far away for convenient scrolling. Thankfully, most KDE apps (including Okular) implement a IPC interface using qdbus. I realized that with a command line interface to Okular, my phone could (via SSH) be used as a remote control.
This turned out to be quite straightforward. The qdbus command (with no arguments) provides a list of all applications that have registered with it. By filtering for 'okular', we can find get an interface to all running Okular instances.
Okular implements a currentDocument method, so we can find which instance has the relevant document. We can then send any number of signals, most usefully slotNextPage and slotPreviousPage
To read a document, I then open it regularly, SSH to my desktop, and send okular_ctl filename slotNextPage whenever I want to see the next page.