Merge PDFs

using OnePdf;
 
string[] files = ["chapter-1.pdf", "chapter-2.pdf"];
List<PdfDocument> sources = files.Select(f => PdfDocument.Open(f)).ToList();
try
{
    using PdfDocument merged = PdfDocument.Merge(sources);
    merged.SetInfo("Title", "Collected chapters");     // Merge copies no metadata
    merged.Save("merged.pdf");
 
    Console.WriteLine($"{merged.PageCount} pages");
    foreach (PdfOutlineItem bookmark in merged.Outline)
        Console.WriteLine($"  bookmark: {bookmark.Title}");
}
finally
{
    foreach (PdfDocument source in sources)
        source.Dispose();
}
 
// PDFs held in memory: open the bytes, merge, and get bytes back.
byte[] first = File.ReadAllBytes("chapter-1.pdf");
byte[] second = File.ReadAllBytes("chapter-2.pdf");
using (PdfDocument a = PdfDocument.Open(first))
using (PdfDocument b = PdfDocument.Open(second))
using (PdfDocument both = PdfDocument.Merge([a, b]))
{
    var buffer = new MemoryStream();
    both.Save(buffer);
    Console.WriteLine($"{buffer.Length:N0} bytes in memory");
}
 
// Append only the first page of another file to an existing document.
using (PdfDocument target = PdfDocument.Open("chapter-1.pdf", new PdfOpenOptions { AllowEditing = true }))
using (PdfDocument other = PdfDocument.Open("chapter-2.pdf"))
{
    target.ImportPages(other, [0]);
    target.Save("chapter-1-plus.pdf");
    Console.WriteLine($"chapter-1-plus.pdf: {target.PageCount} pages");
}

4 pages
  bookmark: Chapter 1, part 1
  bookmark: Chapter 1, part 2
  bookmark: Chapter 2, part 1
  bookmark: Chapter 2, part 2
32,147 bytes in memory
chapter-1-plus.pdf: 3 pages