Q: How do I set the initial page that should be opened when a PDF
document is opened?
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A: You can use prefs.SetInitialPage(dest) in the PDFViewPrefs class.
For example,
....
PDFDocViewPrefs prefs = pdfdoc.GetViewPrefs();
Destination dest_page = Destination.CreateFit(page);
// = Destination.CreateFit(pdfdoc.GetPage(page_num));
prefs.SetInitialPage(dest_page);