iTextSharp PdfStamper add Header/Footer - vb.net

Question

I am trying to add a header or footer to pages within a pdf document. This is explained in the iTextInAction book as the correct way to add direct content to a page. However when I try to open this document in Adobe I get the following error, and have some issues printing as well. Any ideas?
Dim reader As PdfReader = Nothing
Dim stamper As PdfStamper = Nothing
Try
reader = New PdfReader(inputFile)
stamper = New PdfStamper(reader, New IO.FileStream(outputFile, IO.FileMode.Append))
Dim fontSz As Single = 10.0F
Dim font As New Font(font.FontFamily.HELVETICA, fontSz, 1, BaseColor.GRAY)
Dim chunk As New Chunk(headerText, font)
Dim rect As Rectangle = reader.GetPageSizeWithRotation(1)
Here I am just adjusting the size of the text to make sure it fits within the page boundaries
While chunk.GetWidthPoint() > rect.Width
fontSz -= 1.0F
font = New Font(font.FontFamily.HELVETICA, fontSz, 1, BaseColor.GRAY)
chunk = New Chunk(wm.ToString(), font)
End While
This is where I get the overcontent and add my text to it
For pageNo As Int32 = 1 To reader.NumberOfPages
Dim phrase As New Phrase(chunk)
Dim x As Single = (rect.Width / 2) - (phrase.Chunks(0).GetWidthPoint() / 2)
Dim y As Single = If(wm.WatermarkPosition = "Header", rect.Height - font.Size, 1.0F)
Dim canvas As PdfContentByte = stamper.GetOverContent(pageNo)
canvas.BeginText()
ColumnText.ShowTextAligned(canvas, Element.ALIGN_LEFT, phrase, x, y, 0)
canvas.EndText()
Next
Catch ex As iTextSharp.text.pdf.BadPasswordException
Throw New InvalidOperationException("Page extraction is not supported for this pdf document. It must be allowed in order to add a watermark.")
Finally
reader.Close()
stamper.Close()
End Try

Answer 1

You're problem is probably this line:
stamper = New PdfStamper(reader, New IO.FileStream(outputFile, IO.FileMode.Append))
You are telling .Net to write the contents to a file in append mode. If the file doesn't exist then it creates the file but subsequent writes go to the end producing a corrupt PDF. You should change this to IO.FileMode.Create
Also, while you're at it, I usually recommend being even more explicit with your FileStream creation and tell .Net (and thus Windows) what you further intend for the stream. In this case, you are only ever going to write to it to you can say FileAccess.Write and while you are writing to it you want to make sure no one else attempts to read from it (since it would be in an invalid state) so you can say FileShare.None
stamper = New PdfStamper(reader, New FileStream(outputFile, FileMode.Create, FileAccess.Write, FileShare.None))
(An aside, although using IO.FileMode.Create is absolutely valid it is really weird to see. Most people either spell it out as System.IO.FileMode.Create or they import System.IO and then just us FileMode.Create.)