Merge PDF files in C#: files, streams and folders

Build it step by step.

1. 1Install the NuGet packages

   SelectPdf.Universal is the library. The native package carries the Chromium engine for one platform: add the one for the platform you run on.

   Windows x64 Linux x64 Linux ARM64 macOS · Apple Silicon
   terminal COPY

   dotnet add package SelectPdf.Universal
   dotnet add package SelectPdf.Universal.Native.win-x64

   terminal COPY

   dotnet add package SelectPdf.Universal
   dotnet add package SelectPdf.Universal.Native.linux-x64

   terminal COPY

   dotnet add package SelectPdf.Universal
   dotnet add package SelectPdf.Universal.Native.linux-arm64

   terminal COPY

   dotnet add package SelectPdf.Universal
   dotnet add package SelectPdf.Universal.Native.osx-arm64

2. 2Create a console app

   Any .NET 8 or .NET 10 project works. To follow along, create a console app, run the install commands above inside its folder, then replace Program.cs with the code below.

   terminal COPY

   dotnet new console -n PdfSample
   cd PdfSample

3. 3Create two PDFs to merge

   Program.cs · lines 3–10 COPY

   // two source PDFs
   HtmlToPdf converter = new HtmlToPdf();
   PdfDocument a = converter.ConvertHtmlString("<h1>Cover letter</h1>");
   a.Save("a.pdf");
   a.Close();
   PdfDocument b = converter.ConvertHtmlString("<h1>Contract</h1>");
   b.Save("b.pdf");
   b.Close();

   To keep the sample self-contained, HtmlToPdf produces a.pdf and b.pdf. In your application these are the files you already have.

4. 4Append them into a new document

   Program.cs · lines 12–15 COPY

   // merge: append both files into a new document
   PdfDocument merged = new PdfDocument();
   merged.Append("a.pdf");
   merged.Append("b.pdf");

   Start with an empty PdfDocument and call Append once per file, in the order the pages should appear. Append also accepts a stream or another PdfDocument.

5. 5Save the merged file

   Program.cs · lines 16–17 COPY

   merged.Save("merged.pdf");
   merged.Close();

   Save writes merged.pdf with the pages of both inputs; Close releases it.

6. 6Run it

   terminal COPY

   dotnet run

   The program writes merged.pdf to the working folder. Without a license key, every page carries a trial watermark; set GlobalProperties.LicenseKey before the first conversion to remove it.

The complete Program.cs.

This is the exact file our samples pipeline compiled and ran on Linux ARM64 and Linux x64 and macOS ARM64 and Windows x64 to produce the output on this page.

using SelectPdf.Universal;

// two source PDFs
HtmlToPdf converter = new HtmlToPdf();
PdfDocument a = converter.ConvertHtmlString("<h1>Cover letter</h1>");
a.Save("a.pdf");
a.Close();
PdfDocument b = converter.ConvertHtmlString("<h1>Contract</h1>");
b.Save("b.pdf");
b.Close();

// merge: append both files into a new document
PdfDocument merged = new PdfDocument();
merged.Append("a.pdf");
merged.Append("b.pdf");
merged.Save("merged.pdf");
merged.Close();

Open the project on GitHub → this program and the variations below, ready to dotnet run

The PDF it produces.

Variations you will need next.

Each variation is a separate file in the same project, compiled and run with the program above.

Merge PDFs from byte arrays or streams

Uploads, database columns and blob storage hand you bytes, not files. Wrap each one in a MemoryStream and pass it to Append. Keep the streams open until Save has run. Save() without a file name returns the merged PDF as a byte[], ready to send as an HTTP response.

using SelectPdf.Universal;

static class MergeInMemory
{
    public static void Run()
    {
        // PDFs that arrive as bytes: uploads, a database column, a blob store
        List<byte[]> files = [File.ReadAllBytes("a.pdf"), File.ReadAllBytes("b.pdf")];

        PdfDocument merged = new PdfDocument();
        List<MemoryStream> streams = [];
        foreach (byte[] file in files)
        {
            MemoryStream stream = new MemoryStream(file);
            streams.Add(stream);
            merged.Append(stream);
        }

        // Save() without a file name returns the merged PDF as bytes, ready for an HTTP response
        byte[] result = merged.Save();
        merged.Close();
        streams.ForEach(s => s.Dispose());

        File.WriteAllBytes("merged-in-memory.pdf", result);
    }
}

Merge converted HTML without temporary files

ConvertHtmlString returns a PdfDocument, and Append takes one directly, so a cover page and a body produced by separate conversions merge without touching the disk. Close the source documents after the merged one is saved, not before.

using SelectPdf.Universal;

static class MergeConvertedPages
{
    public static void Run()
    {
        // convert straight into documents, no temporary files
        HtmlToPdf converter = new HtmlToPdf();
        PdfDocument cover = converter.ConvertHtmlString("<h1>Annual report</h1><p>Cover page</p>");
        PdfDocument body = converter.ConvertHtmlString("<h2>Results</h2><p>Revenue grew in every region.</p>");

        PdfDocument report = new PdfDocument();
        report.Append(cover);
        report.Append(body);
        report.Save("report.pdf");

        // appended documents must stay open until the merged document is saved
        report.Close();
        cover.Close();
        body.Close();
    }
}

Merge every PDF in a folder

Directory.GetFiles lists the PDFs and Order puts them in file name order, so a numeric or date prefix in the file names controls the page order of the result.

using SelectPdf.Universal;

static class MergeFolder
{
    public static void Run()
    {
        // a folder of PDFs (made here so the sample runs on its own)
        Directory.CreateDirectory("inbox");
        HtmlToPdf converter = new HtmlToPdf();
        foreach (string month in new[] { "01-january", "02-february", "03-march" })
        {
            PdfDocument statement = converter.ConvertHtmlString($"<h1>Statement {month}</h1>");
            statement.Save(Path.Combine("inbox", $"{month}.pdf"));
            statement.Close();
        }

        // merge every PDF in the folder, in file name order
        string[] files = Directory.GetFiles("inbox", "*.pdf").Order(StringComparer.Ordinal).ToArray();

        PdfDocument merged = new PdfDocument();
        foreach (string file in files)
            merged.Append(file);

        merged.Save("statements-q1.pdf");
        merged.Close();
    }
}

Common problems and fixes.

Good to know.

• PdfDocument.Append accepts a PdfDocument, a file name or a stream. Source
• A document passed to Append must stay open until the merged document is saved. Source
• For PDF/A output, create the target with new PdfDocument(PdfStandard.PdfA2B) or another level and append to it. Source