Might worth trying: Try sending a WM_VSCROLL message, using SendMessage() (via PInvoke) to the PDF Reader's Handle, when the button is clicked. Should look like this:

SendMessage(adobePDFReaderHandle, WM_VSCROLL, (IntPtr)1, IntPtr.Zero); // this should scroll down

Hopefully the handle of the control is provided...if not, you'll have to search it... Otherwise, try restoring the focus to the Adobe PDF Reader Control before sending the key combination.