node.js - NodeJS: Merge two PDF files into one using the buffer obtained by reading them

Question

I am using fill-pdf npm module for filling template pdf's and it creates new file which is read from the disk and returned as buffer to callback. I have two files for which i do the same operation. I want to combine the two buffers there by to form a single pdf file which i can send back to the client. I tried different methods of buffer concatenation. The buffer can be concatenated using Buffer.concat, like,

var newBuffer = Buffer.concat([result_pdf.output, result_pdf_new.output]);

The size of new buffer is also the sum of the size of the input buffers. But still when the newBuffer is sent to client as response, it shows only the file mentioned last in the array.

res.type("application/pdf");
return res.send(buffer);

Any idea ?

Answer 1

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);