Introduction

public static void MergeMultipleDocuments()
{
  Console.WriteLine( "=== MERGE MULTIPLE DOCUMENTS ===" );

  var file1 = "Animals_Pages_1_to_3.pdf";
  var file2 = "Animals_Pages_4_to_6.pdf";
  var file3 = "Animals_Pages_7_to_10.pdf";
  var outputFileName = "Merged_Entire_Pdf.pdf";
  var outputPath = MergeSample.MergeOutputDirectory + outputFileName;

  // Load file1, file2 and file3.
  using( var doc1 = PdfDocument.Load( MergeSample.MergeSampleResourceDirectory + file1 ) )
  {
    using( var doc2 = PdfDocument.Load( MergeSample.MergeSampleResourceDirectory + file2 ) )
    {
      using( var doc3 = PdfDocument.Load( MergeSample.MergeSampleResourceDirectory + file3 ) )
      {
        // Create the ouput document.
        using( var mergedDoc = PdfDocument.Create( outputPath ) )
        {
          // Set the order of merge for doc1, doc2 and doc3.
          var docsToMerge = new List<PdfDocument> { doc1, doc2, doc3 };
          // Merge the 3 documents in the empty mergedDoc.
          mergedDoc.Merge( new MergeOptions( docsToMerge ) );

          // Save the output document.
          mergedDoc.Save();
          Console.WriteLine( "Merged multiple documents saved to: " + outputPath );
        }
      }
    }
  }
}