Merge

import com.spire.pdf.PdfDocument;
import com.spire.pdf.PdfDocumentBase;

public class MergePdfsDemo {

    public void mergePdfs(String []pdfFiles){
        String outputFile="output.pdf";

        PdfDocumentBase documentBase= PdfDocument.mergeFiles(pdfFiles);
        documentBase.save(outputFile);

        documentBase.close();
    }
}

using Spire.Pdf;
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace DemoOnlineCode
{
    class MergePdfsDemo
    {
        public void mergePdfs(string[] pdfFiles)
        {
            String outputFile = "output.pdf";

            PdfDocumentBase documentBase = PdfDocument.MergeFiles(pdfFiles);
            documentBase.Save(outputFile);

            documentBase.Close();
        }
    }
}