Principles of Merging PDF and Data with VB.net, and the FDF Toolkit .net component. 

Rules: Make sure when posting PDF Data to a buffer, do not post any HTML along with PDF Data. Remove any header information by declaring "Response.Clear()" before posting any PDF or PDF data MIME content.

Notes: Flattening the pdf form can be set with the true or false switch in the FDFDoc.PDFMergeFDF2Buf function using the FDF Toolkit .net component.

Imports FDFApp
Imports FDFApp.FDFApp_Class
Imports FDFApp.FDFDoc_Class

Public Class loadform
        Inherits System.Web.UI.Page
Private URLPath as String = ""
Private Sub Page_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
    URLPath = Request.Url.ToString
    Try
        Load_FDF()
    Catch Ex as Exception
        Response.Write("Error: " & Ex.Message)
        Response.End
    End Try
End Sub

Public Function Load_FDF()
    ' DECLARE THE COMPONENTS
    Dim FDFApp As New FDFApp.FDFApp_Class
    Dim FDFDoc As New FDFApp.FDFDoc_Class
    ' INITIALIZE THE COMPONENTS
    FDFDoc = FDFApp.FDFCreate
        Response.Clear()
        Response.ContentType = FDFApp.MimePDF

    ' SET TEXT FIELD VALUES
    FDFDoc.FDFSetValue("txtForm_Field1","Field_Value")
    FDFDoc.FDFSetValue("txtForm_Field2","Field_Value")
    FDFDoc.FDFSetValue("txtForm_Field3","Field_Value")

    ' SET OPTION OR LIST FIELD NAMES AND VALUES
    FDFDoc.FDFSetOpt("optForm_Field",New String() _{"Opt_Value01","Opt_Value02","Opt_Value03","Opt_Value04"},New String() _{"DisplayName_01","DisplayName_02","DisplayName_03","DisplayName_04"})
    
    FDFDoc.FDFSetValues("optForm_Field",New String(){"Opt_Value02","Opt_Value03")
    'FDFDoc.FDFSetValue("optForm_Field","Opt_Value02")
    
    ' SET PDF LOCATION
    FDFDoc.FDFSetFile(URLPath & "PDFs/sample.pdf")
    ' MERGE PDF WITH FDF DATA AND FLATTEN
    FDFDoc.PDFData = FDFDoc.PDFMergeFDF2Buf(FDFDoc, True)     ' WRITE THE FDF TO THE BUFFER
    Response.BinaryWrite(FDFDoc.PDFData)

    ' BROWSER READS THE PDF DOCUMENT AND ADOBE PLUG-IN OPENS
    '
End Function

End Class