You have some options.
First things first, Screenshot Captor might not be your best bet -- you could possibly more easily script MiniCap
, the commandline brother of Screenshot Captor.
That still leaves you having to script something that can send PgDown to your pdf reader window. AutoHotKey would be a good solution and there are some expert AHK scripters here that could help.
Another alternative to MiniCap would be my Automatic Screenshotter
tool. Configured properly you wouldn't even ever have to trigger the captures since they would happen automatically every time the window contents changed. With it running you might even find that you could simply press PgDown yourself manually repeatedly since that's all you'd ever have to do.
But again, AHK would make easy work of sending the PgDown key to the reader repeatedly.