Ajax fails to pass html page as parameter to code behind

I am working on a task to convert a webpage to pdf file using SelectPdf. SelectPdf does not support dynamic pages. So I want to use Ajax to pass the webpage as html.

For some reason when I passed ordinary string it works but when I change to use a variable (with html as value) it doesn't. I don't know whether the html content is too large, however I tried with less content and still the same issue. Any help will be appreciated.

The project is language is VB.Net, the page is vbhtml and code behind is a controller.

Please see below the code I have implemented:

VIEW

           var btn = $('#BtnCreateHtmlToPdf');

           btn.click(function () {

            var theHtml = document.documentElement.innerHTML;

            //Just to see there is a value
            alert(theHtml)  

            $(function () {
                $.ajax({
                    type: 'post',
                    url: "/CreatHtmlToPdf/CreatePdf",
                    dataType: "html",
                    data: { HTML: theHtml }
                })
                .done(function (results) {
                    alert("Html data: " + results);
                });
            });
           });

CODE BEHIND

Public Class CreatHtmlToPdfController
    Inherits Controller

    ' GET: CreatHtmlToPdf
    Function Index() As ActionResult

        Return View()
    End Function

    <HttpPost()>
    Function CreatePdf(ByVal HTML As String) As ActionResult

        Dim doc As PdfDocument

        ' read parameters from the webpage
        Dim htmlString As String = HTML

        ' instantiate a html to pdf converter object
        Dim converter As New HtmlToPdf()

        ' create a new pdf document converting an url
        If (HTML <> String.Empty) Then

            doc = converter.ConvertHtmlString(htmlString)


        End If


        ' save pdf document
        Dim pdf As Byte() = doc.Save()

        ' close pdf document
        doc.Close()

        ' return resulted pdf document
        Dim fileResult As FileResult = New FileContentResult(pdf, "application/pdf")
        fileResult.FileDownloadName = "Results_page.pdf"
        Return fileResult

    End Function

    'Declaration
    'Public Property EnablePageMethods As Boolean

End Class