This demo uses the DevExpress PDF Document API to merge multiple PDFs into a single file. You can merge predefined sample files or supply your own documents. To do the latter, select Upload a File in the file selection drop-down menu and upload files one at a time.
Click the Merge Documents button to merge files and download the result.
Sample document
Sample_Main.pdf
Sample_Alternative.pdf
using DevExpress.Pdf;
Stream MergeDocuments(IEnumerable<Stream> documentStreams) {
using var processor = new PdfDocumentProcessor();
processor.CreateEmptyDocument();
foreach(var stream in documentStreams)
processor.AppendDocument(stream);
var outputStream = new MemoryStream();
processor.SaveDocument(outputStream);
outputStream.Position = 0;
return outputStream;
}