Introduce events similar to the PageRenderStarted and PageRenderCompleted ones that the Telerik UI for WinForms RadPdfViewer provides:
WinForms PdfViewer How-To Handle Rendering Events - Telerik UI for WinForms
The attached image shows the issue.
var pageMarginsPropInfo = typeof(PagesLayoutManagerBase).GetField("pageMargins", System.Reflection.BindingFlags.Static | System.Reflection.BindingFlags.NonPublic);
pageMarginsPropInfo.SetValue(null, new Size(0, 0));