Here is the code to merge a pdf file in C#.

namespace MergePDF {
        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
        }
    }

So you can call the merge function in the context.

string sourcePath = @"D:\Pdf\";
                MergeEx objMergeEx = new MergeEx();
                objMergeEx.SourceFolder = sourcePath;
                objMergeEx.DestinationFile = sourcePath + "Mergedfile.pdf";
                objMergeEx.AddFile("parsetest.pdf");
                objMergeEx.AddFile(@"regions\" + "eisacktal.pdf");
                objMergeEx.Execute();

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