getting error in PdfCopy copy = new PdfCopy(document, stream);

I know that you are using the iTextSharp Library to achieve this functionality, but I wanted to note that the LEADTOOLS SDK can help you acheive this as well. You can use LEADTOOLS Document SDK technology in your application.

https://www.leadtools.com/sdk/document

You can leverage the LEADDocument class to load, clone, and export document files. See the below code:

// Setup a cache 
FileCache cache = new FileCache();
cache.CacheDirectory = @"c:\cache-dir";

byte[] bytes = File.ReadAllBytes(@"Image File Path");
MemoryStream ms = new MemoryStream(bytes);

ms.Position = 0;
LEADDocument _orignal = DocumentFactory.LoadFromStream(ms, new LoadDocumentOptions { Cache = cache });
LEADDocument _orignalCopy = _orignal.Clone(cache, new CloneDocumentOptions());

Stream _stream = _orignalCopy.Stream;

return _stream;

public static MemoryStream MergePdfStreams(params MemoryStream[] pdfStreams)
{
    MemoryStream mergedStream = new MemoryStream();
    Document document = new Document();
    MemoryStream stream = new MemoryStream();

    // Initialize the PdfCopy object with the correct document and stream
    PdfCopy copy = new PdfCopy(document, stream);

    document.Open();

    foreach (MemoryStream pdfStream in pdfStreams)
    {
        PdfReader reader = new PdfReader(pdfStream.ToArray());
        for (int i = 1; i <= reader.NumberOfPages; i++)
        {
            PdfImportedPage page = copy.GetImportedPage(reader, i);
            copy.AddPage(page);
        }
        reader.Close();
    }

    document.Close();
    return stream;
}

public static MemoryStream MergePdfStreams(params MemoryStream[] pdfStreams)
{
    MemoryStream mergedStream = new MemoryStream();
    Document document = new Document();
    MemoryStream stream = new MemoryStream();
    
    // Initialize the PdfCopy object with the correct document and stream
    PdfCopy copy = new PdfCopy(document, stream);
    
    document.Open();

    foreach (MemoryStream pdfStream in pdfStreams)
    {
        PdfReader reader = new PdfReader(pdfStream.ToArray());
        for (int i = 1; i <= reader.NumberOfPages; i++)
        {
            PdfImportedPage page = copy.GetImportedPage(reader, i);
            copy.AddPage(page);
        }
        reader.Close();
    }

    document.Close();
    return stream;
}

GETTING System.NullReferenceException: 'Object reference not set to an instance of an object.

of this line

PdfCopy copy = new PdfCopy(document, stream);

after adding this -

if (document != null) {

PdfCopy copy = new PdfCopy(document, stream);

}

then getting issue in - can't find copy valirable

PdfImportedPage page = copy.GetImportedPage(reader, i);
                    copy.AddPage(page);

What is the error message?

Update:
Sorry, I can see error message

Suspect:
I suspect, your document is null. to confirm this can you add if a condition like below and initialise pdfcopy

if (document != null) {

PdfCopy copy = new PdfCopy(document, stream);

}

If document is null, then that's the issue