Recently I had to create an assembly component using C# to merge multiple PDF files into one file. The specification was pretty straight forward:
1) Merge two of more PDF document into a single output PDF File.
2) Used on an ASP.NET Page

After a few minutes of GOOG, I came up to this “ITextSharp”

iText# (iTextSharp) is a port of the iText open source java library written entirely in C# for the .NET platform. iText# is a library that allows you to generate PDF files on the fly. It is implemented as an assembly.

It must be noted that the assembly is coded and compiled using the .NET Framework 1.1. You might want to migrate to the version 2.0 or 3.5 of the .NET Framework.

With a some more research on PDF merging. I was able to create a class that will make use of the ITextSharp assembly and perform as the merge pdf operation as needed.

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Here is the code:

using iTextSharp.text;
using iTextSharp.text.pdf;

public class MergeEx
{
#region Fields
private string sourcefolder;
private string destinationfile;
private IList fileList = new ArrayList();
#endregion

#region Public Methods
///
/// Add a new file, together with a given docname to the fileList and namelist collection
///
///  public void AddFile(string pathnname)
{
fileList.Add(pathnname);
}

///
/// Generate the merged PDF
///
public void Execute()
{
MergeDocs();
}
#endregion

#region Private Methods
///
/// Merges the Docs and renders the destinationFile
///
///  ///  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(); }
}
#endregion

#region Properties
///
/// Gets or Sets the SourceFolder
///
public string SourceFolder
{
get { return sourcefolder; }
set {sourcefolder = value; }
}

///
/// Gets or Sets the DestinationFile
///
public string DestinationFile
{
get { return destinationfile; }
set { destinationfile = value; }
}
#endregion
}

To use the MergeEx class:
1) Initialize the class
2) Set the SourceFolder and DestinationFile properties
3) Using the AddFile method, add the source file names that need to be merged (Filename only since the SourceFolder has already been set)
4) Call the Execute Method

If everything works fine you will find your Merged PDF Document at your stated destination file.

The code is pretty much self decribed. If there is any question, i can always be contacted via this post.