iText7 for .NET - error while merging files - c#-4.0

Question

I've created simple method that allows me to merge multiple PDF files.
Below is my code:
private void Merge(List<string> src, string dest)
{
Stopwatch sw = new Stopwatch();
sw.Start();
PdfDocument pdfDocument1 = new PdfDocument(new PdfReader(src[0]), new PdfWriter(dest));
for (int i = 1,max=src.Count; i < max; i++)
{
PdfDocument pdfDocument2 = new PdfDocument(new PdfReader(src[i]));
var pagesCount = pdfDocument2.GetNumberOfPages();
pdfDocument2.CopyPagesTo(1, pagesCount, pdfDocument1);
pdfDocument2.Close();
}
pdfDocument1.Close();
sw.Stop();
Debug.WriteLine(sw.Elapsed);
}
I've based my code on example from iText book: http://developers.itextpdf.com/examples/merging-pdf-documents/clone-merging-documents-bookmarks
For test purpose I've attached that method to button and I'm caling it like this:
private void button1_Click(object sender, EventArgs e)
{
try
{
string dest = #"E:\final.pdf";
var files = Directory.GetFiles(#"E:\PDFS", "*.pdf").Take(100).ToList();
Merge(files,dest);
}
catch (Exception exception)
{
Console.WriteLine(exception);
}
}
From time to time (after n'th click on that button) I get exception saying:
Cant cast from 'iText.Kernel.Pdf.PdfNumber' to
'iText.Kernel.Pdf.PdfStream'.
Also first time I click my application takes about 100MB of memory, next time I click it increase to 150MB, next click and I get 230MB of memory used, so it look like it isn't releasing memory.
Is there a better way to merge multiple PDF's into one using iTextSharp 7?
As per request I'm adding StackTrace:
w iText.Kernel.Pdf.PdfPage.GetContentStream(Int32 index)
w iText.Kernel.Pdf.PdfPage.Flush(Boolean flushXObjects)
w iText.Kernel.Pdf.PdfPage.Flush()
w iText.Kernel.Pdf.PdfDocument.Close()
w iTextSharp7_Merge.Form1.Merge(List`1 src, String dest)
w c:\Users\Misiu\Documents\Visual Studio 2013\Projects\iTextSharp7_Merge\Form1.cs:wiersz 75
And here are exception details:
EDIT:
I've changed button click function so now it loads 100 file names from directory and calls Merge method 10 times with same list:
private void button1_Click(object sender, EventArgs e)
{
try
{
string dest = #"E:\final.pdf";
var files = Directory.GetFiles(#"E:\PDFS", "*.pdf").OrderBy(x => x).Skip(0).Take(100).ToList();
Stopwatch sw = new Stopwatch();
sw.Start();
for (int i = 1; i <= 10; i++)
{
Debug.WriteLine(i);
Merge(files, dest1);
}
sw.Stop();
Debug.WriteLine(sw.Elapsed);
}
catch (Exception exception)
{
Console.WriteLine(exception);
}
}
This way I've excluded random sort of Directory.GetFiles.
Here is sample output that comes from Visual Studio:
'iTextSharp7_Merge.vshost.exe' (Managed (v4.0.30319)): Loaded 'C:\WINDOWS\Microsoft.Net\assembly\GAC_MSIL\System.Configuration\v4.0_4.0.0.0__b03f5f7f11d50a3a\System.Configuration.dll'
1
'iTextSharp7_Merge.vshost.exe' (Managed (v4.0.30319)): Loaded 'C:\Users\Misiu\documents\visual studio 2013\Projects\iTextSharp7_Merge\bin\Debug\itext.kernel.dll'
'iTextSharp7_Merge.vshost.exe' (Managed (v4.0.30319)): Loaded 'C:\Users\Misiu\documents\visual studio 2013\Projects\iTextSharp7_Merge\bin\Debug\itext.io.dll'
A first chance exception of type 'System.NullReferenceException' occurred in itext.kernel.dll
2
3
The thread '<No Name>' (0x2dd0) has exited with code 0 (0x0).
4
5
A first chance exception of type 'System.InvalidCastException' occurred in itext.kernel.dll