The Simple is Core. Thanks.
OK. I change your simple project. So one form open second form named dlgPdfViewer with PdfViewerControl put on it by VS Designer. (The problem I reported not happened if PDF load again in same Form)
All work fine.
For some reason my Original project still show spinning. I not have match time to find out the differences.
But for while I work around by use Static form to all project, and show it again when need, instead open New form each time. Or i use with code add
PdfViewerControl without the VS
Designer.
Thanks