iText CopyAsFormXObject PdfException

Question

As I am processing my main pdf file and when I get to page 2 I start to read this Adobe Illustrator. I get this PdfException at the line origPage.CopyAsFormXObject(aiDocument); about the indirect object. I am not sure why I am getting this error because its a different file.

iText.Kernel.Exceptions.PdfException: 'Cannot copy indirect object from the document that is being written.'

const float imageX = 111.318f, imageY = 130.791f;
//const float imageWidth = 755.454f, imageHeight = 432.094f;
    
string aifilePath = @"PathToAIFile.ai";    
var buffer = File.ReadAllBytes(aifilePath);
using (var aiStream = new MemoryStream(buffer))
using (var aimodifiedaiStream = new MemoryStream())
{
   var aiReader = new iText.Kernel.Pdf.PdfReader(aiStream);
   var aiDocument = new PdfDocument(aiReader, new PdfWriter(aimodifiedaiStream));
   PdfPage origPage = aiDocument.GetFirstPage();
   PdfFormXObject  aifForm = origPage.CopyAsFormXObject(aiDocument);
   canvasPage.AddXObjectAt(aifForm, imageX, imageY);
}

Updated with two PdfDocument

const float imageX = 111.318f, imageY = 130.791f;
//const float imageWidth = 755.454f, imageHeight = 432.094f;

string aifilePath = @"PathToAIFile.ai";    
var buffer = File.ReadAllBytes(aifilePath);
using (var aiStream = new MemoryStream(buffer))
using (var aimodifiedaiStream = new MemoryStream())
{
   var aisourceReader = new iText.Kernel.Pdf.PdfReader(aiStream);
   var pdfsourceDocument = new PdfDocument(aisourceReader, new PdfWriter(aimodifiedaiStream));
   var pdftargetDocument = new PdfDocument(aisourceReader);
   PdfPage origPage = aiDocument.GetFirstPage();
   PdfFormXObject  aifForm = origPage.CopyAsFormXObject(aiDocument);
   canvasPage.AddXObjectAt(aifForm, imageX, imageY);
}

Updated with pdftargetDocument has a writer and pdfsourceDocument with just PdfReader still getting the error Cannot copy to document opened in reading mode.'

using (var aiStream = new MemoryStream(buffer))
using (var aimodifiedaiStream = new MemoryStream())
   {
   var aisourceReader = new iText.Kernel.Pdf.PdfReader(new MemoryStream(buffer));
   var aitargetReader = new iText.Kernel.Pdf.PdfReader(new MemoryStream(buffer));
                
   var pdfsourceDocument = new PdfDocument(aisourceReader);
   var pdftargetDocument = new PdfDocument(aitargetReader, new PdfWriter(aimodifiedaiStream));
   PdfPage origPage = pdfsourceDocument.GetFirstPage();

   PdfFormXObject  aifForm = origPage.CopyAsFormXObject(pdfsourceDocument);
   canvasPage.AddXObjectAt(aifForm, imageX, imageY);
   pdfsourceDocument.Close();
   pdftargetDocument.Close();
   }

Answer 1

Summing up my comments as an answer...

You copy a page from the PdfDocument aiDocument as form XObject to the same aiDocument.

Clearly aiDocument is being written, it has a PdfWriter in its constructor. But the iText API only allows copying pages from a PdfDocument constructed without a PdfWriter.

Furthermore, you can only copy to a PdfDocument that is written to, i.e. that is constructed with a PdfWriter. Thus, you cannot copy from a document into itself.

Thus, you need different PdfDocument instances as source and target of the copy process.

The source PdfDocument instance is generated with only a PdfReader, no PdfWriter. The target PdfDocument instance is generated with a PdfWriter. If the target also is generated with a PdfReader depends on your precise use case: Do you want to add the XObject to an existing document, or do you want to add it to an empty document without any prior content.