But I think I've done that when I select the webcontrol with my mouse...
when the gotfocus event is triggered the control will have focus, so a hwnd will exist. The control will have focus after a click, doubleclick, keydown or mousedown event. Or vba code to setfocus. Mousemove is insufficient on its own, which is why I said you need to setfocus with the mousemove event.
Not at all clear to me what you are trying to achieve other than detecting when a user clicks on predefined area of pdf when displayed in a web control. It would appear the whole pdf cannot be displayed, hence the need for feedback from the scrollbar to determine how far the pdf has been scrolled - so you can calculate the offset to the predefined area.
You still need to determine the coordinates for the predefined area.
Once you have done that, use the mousedown or up event to check whether the cursor is within the (offset) area and do whatever it is required. You cannot use the click, dblclick or key events because they don't return the mouse X,Y coordinates. To use those events you need additional code to get the cursor position - see the getcursorpos api. It returns a value in pixels, which you need to a) convert to twips and b) then offset for the access>form>control position. For the latter, check out the ScreenToClient api