PdfHtmlHeaderFooterReserveSpaceAlways Property

If true, space is reserved on all pages regardless of header and footer visibility. This is the default behavior. If false, space for the header or footer is reserved only on pages where they are actually rendered; the other pages keep the margins of the document. It is ignored if ShowOnlyInHtmlToPdfPages is explicitly set to false. The default value is true

public bool ReserveSpaceAlways { get; set; }