The behavior of the HTML to PDF converter is it will fit the content to the width of the page by default. So this leads to the large space below the HTML content. To overcome this we suggest you to increase the width of the PDF document or you can change the Viewport size accordingly. We have attached the sample for your reference.
Please find the sample code example :
WebKitConverterSettings webKitSettings = new WebKitConverterSettings();
webKitSettings.Orientation = PdfPageOrientation.Landscape;
//set view port
webKitSettings.WebKitViewPort = new Size(1200, 1200);
webKitSettings.WebKitPath = @"../../QtBinaries";
Please refer to the following sample:
Please try this and let us know if you need any further assistance.