Unable to merge 2 PDFs using MemoryStream

This answer from Stack Overflow (Combine two (or more) PDF's) by Andrew Burns works for me:

        using (PdfDocument one = PdfReader.Open("pdf 1.pdf", PdfDocumentOpenMode.Import))
        using (PdfDocument two = PdfReader.Open("pdf 2.pdf", PdfDocumentOpenMode.Import))
        using (PdfDocument outPdf = new PdfDocument())
        {
            CopyPages(one, outPdf);
            CopyPages(two, outPdf);

            outPdf.Save("file1and2.pdf");
        }

        void CopyPages(PdfDocument from, PdfDocument to)
        {
            for (int i = 0; i < from.PageCount; i++)
            {
                to.AddPage(from.Pages[i]);
            }
        }

Merging pdf in C# or any other language is not straight forward with out using 3rd party library.

I assume your requirement for not using library is that most Free libraries, nuget packages has limitation or/and cost money for commercial use.

I have made research and found you an Open Source library called PdfClown with nuget package, it is also available for Java. It is Free with out limitation (donate if you like). The library has a lot of features. One such you can merge 2 or more documents to one document.

I supply my example that take a folder with multiple pdf files, merged it and save it to same or another folder. It is also possible to use MemoryStream, but I do not find it necessary in this case.

The code is self explaining, the key point here is using SerializationModeEnum.Incremental:

public static void MergePdf(string srcPath, string destFile)
{
    var list = Directory.GetFiles(Path.GetFullPath(srcPath));
    if (string.IsNullOrWhiteSpace(srcPath) || string.IsNullOrWhiteSpace(destFile) || list.Length <= 1)
        return;
    var files = list.Select(File.ReadAllBytes).ToList();
    using (var dest = new org.pdfclown.files.File(new org.pdfclown.bytes.Buffer(files[0])))
    {
        var document = dest.Document;
        var builder = new org.pdfclown.tools.PageManager(document);
        foreach (var file in files.Skip(1))
        {
            using (var src = new org.pdfclown.files.File(new org.pdfclown.bytes.Buffer(file)))
            { builder.Add(src.Document); }
        }

        dest.Save(destFile, SerializationModeEnum.Incremental);
    }
}

To test it

var srcPath = @"C:\temp\pdf\input";
var destFile = @"c:\temp\pdf\output\merged.pdf";
MergePdf(srcPath, destFile);

Input examples
PDF doc A and PDF doc B

Output example

Links to my research:

• https://csharp-source.net/open-source/pdf-libraries
• https://sourceforge.net/projects/clown/
• https://www.oipapio.com/question-3526089

Disclaimer: A part of this answer is taken from my my personal web site https://itbackyard.com/merge-multiple-pdf-files-to-one-pdf-file-in-c/ with source code to github.