grazie, ma ho trovato la soluzione

codice:
using iTextSharp.text;
using iTextSharp.text.pdf;

string sourcefolder = Server.MapPath("MyFolder") ;
string destinationfile = "\\ NomeFile" ;

private void MergeDocs()
{
 
//Step 1: Create a Docuement-Object
Document document = new Document();
try
{
//Step 2: we create a writer that listens to the document
PdfWriter writer = PdfWriter.GetInstance(document, new FileStream(destinationfile, FileMode.Create));
 
//Step 3: Open the document
document.Open();
 
PdfContentByte cb = writer.DirectContent;
PdfImportedPage page;
 
int n = 0;
int rotation = 0;
 
//Loops for each file that has been listed
foreach (string filename in fileList)
{
//The current file path
string filePath = sourcefolder + filename;
 
// we create a reader for the document
PdfReader reader = new PdfReader(filePath);
 
//Gets the number of pages to process
n = reader.NumberOfPages;
 
int i = 0;
while (i < n)
{
i++;
document.SetPageSize(reader.GetPageSizeWithRotation(1));
document.NewPage();
 
//Insert to Destination on the first page
if (i == 1)
{
Chunk fileRef = new Chunk(" ");
fileRef.SetLocalDestination(filename);
document.Add(fileRef);
}
 
page = writer.GetImportedPage(reader, i);
rotation = reader.GetPageRotation(i);
if (rotation == 90 || rotation == 270)
{
cb.AddTemplate(page, 0, -1f, 1f, 0, 0, reader.GetPageSizeWithRotation(i).Height);
}
else
{
cb.AddTemplate(page, 1f, 0, 0, 1f, 0, 0);
}
}
}
}
catch (Exception e) { throw e; }
finally { document.Close(); }
}