Where is the HDC handle coming from?

The PAINT event of the BiDisp.ocx pass HDC as parameter. in the BiDisp_Paint(ByVal hDC As Long) method. The AnnoUIOnPaint(hDC) use this HDC to draw annotation objects. See the source below:

Private Sub BiDisp_Paint(ByVal hDC As Long)

…

    Anno.AnnoUIOnPaint (hDC)

…

End Sub