ShowHiddenSheets.cs

// For complete examples and data files, please go to https://github.com/groupdocs-viewer/GroupDocs.Viewer-for-.NET
// Setup GroupDocs.Viewer config
ViewerConfig config = Utilities.GetConfigurations();

// Create ViewerHandler
ViewerHtmlHandler htmlHandler = new ViewerHtmlHandler(config);
// Set guid
string guid = DocumentName;

// Set html options to render hidden sheets (do same while using ImageOptions)
HtmlOptions options = new HtmlOptions();
options.ShowHiddenPages = true;

// Get pages
List<PageHtml> pages = htmlHandler.GetPages(guid, options);

foreach (PageHtml page in pages)
{
    //Save each page at disk
    Utilities.SaveAsHtml(page.PageNumber + "_" + DocumentName, page.HtmlContent);
}