NodeJS: Merge two PDF files into one using the buffer obtained by reading them

As mentioned by @MechaCode, the creator has ended support for HummusJS.

So I would like to give you 2 solutions.

1. Using node-pdftk npm module

   The Following sample code uses node-pdftk npm module to combine two pdf buffers seamlessly.

   const pdftk = require('node-pdftk');
   
   var pdfBuffer1 = fs.readFileSync("./pdf1.pdf");
   var pdfBuffer2 = fs.readFileSync("./pdf2.pdf");
   
   pdftk
       .input([pdfBuffer1, pdfBuffer2])
       .output()
       .then(buf => {
           let path = 'merged.pdf';
           fs.open(path, 'w', function (err, fd) {
               fs.write(fd, buf, 0, buf.length, null, function (err) {
                   fs.close(fd, function () {
                       console.log('wrote the file successfully');
                   });
               });
           });
       });
   

   The requirement for node-pdftk npm module is you need to install the PDFtk library. Some of you may find this overhead / tedious. So I have another solution using pdf-lib library.

2. Using pdf-lib npm module

   const PDFDocument = require('pdf-lib').PDFDocument
   
   var pdfBuffer1 = fs.readFileSync("./pdf1.pdf"); 
   var pdfBuffer2 = fs.readFileSync("./pdf2.pdf");
   
   var pdfsToMerge = [pdfBuffer1, pdfBuffer2]
   
   const mergedPdf = await PDFDocument.create(); 
   for (const pdfBytes of pdfsToMerge) { 
       const pdf = await PDFDocument.load(pdfBytes); 
       const copiedPages = await mergedPdf.copyPages(pdf, pdf.getPageIndices());
       copiedPages.forEach((page) => {
            mergedPdf.addPage(page); 
       }); 
   } 
   
   const buf = await mergedPdf.save();        // Uint8Array
   
   let path = 'merged.pdf'; 
   fs.open(path, 'w', function (err, fd) {
       fs.write(fd, buf, 0, buf.length, null, function (err) {
           fs.close(fd, function () {
               console.log('wrote the file successfully');
           }); 
       }); 
   }); 
   
   

Personally I prefer to use pdf-lib npm module.

Here's what we use in our Express server to merge a list of PDF blobs.

const { PDFRStreamForBuffer, createWriterToModify, PDFStreamForResponse } = require('hummus');
const { WritableStream } = require('memory-streams');

// Merge the pages of the pdfBlobs (Javascript buffers) into a single PDF blob                                                                                                                                                                  
const mergePdfs = pdfBlobs => {
  if (pdfBlobs.length === 0) throw new Error('mergePdfs called with empty list of PDF blobs');
  // This optimization is not necessary, but it avoids the churn down below                                                                                                                                                
  if (pdfBlobs.length === 1) return pdfBlobs[0];

  // Adapted from: https://stackoverflow.com/questions/36766234/nodejs-merge-two-pdf-files-into-one-using-the-buffer-obtained-by-reading-them?answertab=active#tab-top                                                     
  // Hummus is useful, but with poor interfaces -- E.g. createWriterToModify shouldn't require any PDF stream                                                                                                              
  // And Hummus has many Issues: https://github.com/galkahana/HummusJS/issues                                                                                                                                              
  const [firstPdfRStream, ...restPdfRStreams] = pdfBlobs.map(pdfBlob => new PDFRStreamForBuffer(pdfBlob));
  const outStream = new WritableStream();
  const pdfWriter = createWriterToModify(firstPdfRStream, new PDFStreamForResponse(outStream));
  restPdfRStreams.forEach(pdfRStream => pdfWriter.appendPDFPagesFromPDF(pdfRStream));
  pdfWriter.end();
  outStream.end();
  return outStream.toBuffer();
};

module.exports = exports = {
  mergePdfs,
};

HummusJS supports combining PDFs using its appendPDFPagesFromPDF method

Example using streams to work with buffers:

const hummus = require('hummus');
const memoryStreams = require('memory-streams');

/**
 * Concatenate two PDFs in Buffers
 * @param {Buffer} firstBuffer 
 * @param {Buffer} secondBuffer 
 * @returns {Buffer} - a Buffer containing the concactenated PDFs
 */
const combinePDFBuffers = (firstBuffer, secondBuffer) => {
    var outStream = new memoryStreams.WritableStream();

    try {
        var firstPDFStream = new hummus.PDFRStreamForBuffer(firstBuffer);
        var secondPDFStream = new hummus.PDFRStreamForBuffer(secondBuffer);

        var pdfWriter = hummus.createWriterToModify(firstPDFStream, new hummus.PDFStreamForResponse(outStream));
        pdfWriter.appendPDFPagesFromPDF(secondPDFStream);
        pdfWriter.end();
        var newBuffer = outStream.toBuffer();
        outStream.end();

        return newBuffer;
    }
    catch(e){
        outStream.end();
        throw new Error('Error during PDF combination: ' + e.message);
    }
};

combinePDFBuffers(PDFBuffer1, PDFBuffer2);