I will show two methods to flatten editable form fields in a PDF file. Method 1 is true form flattening and method 2 is just making the editable fields read only. Personally I was using method 1 for some time, however in some instances (more specifically when merging images into a PDF file and messing with the editable fields), this specific method would throw an error upon opening the PDF file in Adobe Reader stating that Acrobat may not display the page correctly, however I never noticed anything wrong with the PDF file, everything was being displayed correctly, but this “error” was kinda annoying, so I opted to change the code a bit to make all the fields Read Only instead of flattening the form.

Below is the code to perform both, flatten the form and make all the fields Read Only.

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Method 1

Just create a stamper and set its FormFlattening to true.

C# Code

stamper.FormFlattening = True

VB.Net Code

stamper.FormFlattening = True

Method 2

Create a stamper, read all the AcroFields into an array and loop thru them setting them READ ONLY one by one.

C# Code

string[] fields = stamper.AcroFields.Fields.Select(x => x.Key).ToArray();
for (int key = 0; key <= fields.Count - 1; key++) {
	stamper.AcroFields.SetFieldProperty(fields(key), "setfflags", PdfFormField.FF_READ_ONLY, null);
}

VB.Net Code

Dim fields() As String = stamper.AcroFields.Fields.[Select](Function(x) x.Key).ToArray()

For key As Integer = 0 To fields.Count - 1
  stamper.AcroFields.SetFieldProperty(fields(key), "setfflags", PdfFormField.FF_READ_ONLY, Nothing)
Next

Complete Code

C# Code

private void flattenPdfFile(ref byte[] fileBytes) {
	PdfReader reader = new PdfReader(fileBytes);
	MemoryStream ms = new MemoryStream();
	PdfStamper stamper = new PdfStamper(reader, ms, "4"); //Creating the PDF in version 1.4
	//Method 1
	stamper.FormFlattening = true;

	//Method 1
	string[] fields = stamper.AcroFields.Fields.Select(x =>x.Key).ToArray();
	for (int key = 0; key <= fields.Count - 1; key++) {
		stamper.AcroFields.SetFieldProperty(fields(key), "setfflags", PdfFormField.FF_READ_ONLY, null);
	}

	stamper.Writer.CloseStream = false;
	stamper.Close();
	ms.Position = 0;
	fileBytes = ms.ToArray();

	ms.Flush();
	reader.Close();
}

VB.Net Code

Private Sub flattenPdfFile(ByRef fileBytes As Byte())
  Dim reader As PdfReader = New PdfReader(fileBytes)
  Dim ms As MemoryStream = New MemoryStream()
  Dim stamper As PdfStamper = New PdfStamper(reader, ms, "4") 'Creating the PDF in version 1.4

  'Method 1
  stamper.FormFlattening = True

  'Method 2
  Dim fields() As String = stamper.AcroFields.Fields.[Select](Function(x) x.Key).ToArray()
  For key As Integer = 0 To fields.Count - 1
    stamper.AcroFields.SetFieldProperty(fields(key), "setfflags", PdfFormField.FF_READ_ONLY, Nothing)
  Next

  stamper.Writer.CloseStream = False
  stamper.Close()
  ms.Position = 0
  fileBytes = ms.ToArray()

  ms.Flush()
  reader.Close()
End Sub'

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