redraw

Window redraw event.
[event] redraw([in] LONG dc, [in] int left, [in] int top, [in] int right, [in] int bottom)
This event will be called whenever the XpdfViewer window (or a portion of the window) needs to be redrawn. This can be used to draw arbitrary content on top of the PDF file.

The arguments provide a DC and the region that is being redrawn.

VB6:
Private Sub viewer_redraw(ByVal dc As Long, ByVal Left As Long, ByVal Top As Long, _ ByVal right As Long, ByVal bottom As Long) ' use Windows GDI functions here... End Sub
getWindowPageRange