PdfDocumentOptionsAutoResizePdfPageHeight Property

Automatically resizes the PDF page height to match the HTML content height at the default 96 DPI resolution. When set to true, the final PDF page height is also affected by the HtmlViewerHeight property. To obtain a PDF with the exact page size given by PdfPageSize, set the AutoResizePdfPageWidth and AutoResizePdfPageHeight properties to false. The default value is false

public bool AutoResizePdfPageHeight { get; set; }