PoC to demonstrate how easy it is to merge PDFs. Based on https://stackoverflow.com/a/808699/7342859

mergePDFs.cs

using System;
using System.IO;
using System.Linq;
using PdfSharp.Pdf;
using PdfSharp.Pdf.IO;

namespace MergePDFs
{
    internal class Program
    {
        private static void CopyPages(string inFile, PdfDocument outPDF)
        {
            if (!File.Exists(inFile))
            {
                Console.WriteLine($"{inFile} doesn't exist!");
                return;
            }

            try
            {
                using (var inPDF = PdfReader.Open(inFile, PdfDocumentOpenMode.Import))
                {
                    for (var i = 0; i < inPDF.PageCount; i++)
                    {
                        outPDF.AddPage(inPDF.Pages[i]);
                    }
                }
            }
            catch (Exception e)
            {
                Console.WriteLine($"Problem processing {inFile}:");
                Console.WriteLine(e);
            }
        }
        private static void Main(string[] args)
        {
            if (!args.Any())
            {
                Console.WriteLine("You need to provide path to PDF(s).");
                return;
            }

            using (var outPDF = new PdfDocument())
            {
                foreach (var inFile in args)
                {
                    CopyPages(inFile, outPDF);
                }

                outPDF.Save("./merged.pdf");
            }
        }
    }
}