Is there a way to combine PDFs in pdf.js?

Combining multiple documents and merely displaying them as one with pdf.js is easily possible - i just hacked the following example based on the simple prev/next viewer example that mozilla provides in their repository.

// If absolute URL from the remote server is provided, configure the CORS
// header on that server.
//
var urls = [
  "http://mozilla.github.io/pdf.js/web/compressed.tracemonkey-pldi-09.pdf",
  "http://mozilla.github.io/pdf.js/web/compressed.tracemonkey-pldi-09.pdf",
];

// Disable workers to avoid yet another cross-origin issue (workers need
// the URL of the script to be loaded, and dynamically loading a cross-origin
// script does not work).
//
// pdfjsLib.disableWorker = true;

// In cases when the pdf.worker.js is located at the different folder than the
// pdf.js's one, or the pdf.js is executed via eval(), the workerSrc property
// shall be specified.
//
// pdfjsLib.workerSrc = 'pdf.worker.js';

/**
 * @typedef {Object} PageInfo
 * @property {number} documentIndex
 * @property {number} pageNumber
 */

var pdfDocs = [],
  /**
   * @property {PageInfo}
   */
  current = {},
  totalPageCount = 0,
  pageNum = 1,
  pageRendering = false,
  pageNumPending = null,
  scale = 0.8,
  canvas = document.getElementById("the-canvas"),
  ctx = canvas.getContext("2d");

/**
 * Get page info from document, resize canvas accordingly, and render page.
 * @param num Page number.
 */
function renderPage(num) {
  pageRendering = true;
  current = getPageInfo(num);
  // Using promise to fetch the page
  pdfDocs[current.documentIndex].getPage(current.pageNumber).then(function(page) {
    var viewport = page.getViewport({ scale: scale });
    canvas.height = viewport.height;
    canvas.width = viewport.width;

    // Render PDF page into canvas context
    var renderContext = {
      canvasContext: ctx,
      viewport: viewport,
    };
    var renderTask = page.render(renderContext);

    // Wait for rendering to finish
    renderTask.promise.then(function() {
      pageRendering = false;
      if (pageNumPending !== null) {
        // New page rendering is pending
        renderPage(pageNumPending);
        pageNumPending = null;
      }
    });
  });

  // Update page counters
  document.getElementById("page_num").textContent = pageNum;
}

/**
 * If another page rendering in progress, waits until the rendering is
 * finished. Otherwise, executes rendering immediately.
 */
function queueRenderPage(num) {
  if (pageRendering) {
    pageNumPending = num;
  } else {
    renderPage(num);
  }
}

/**
 * Displays previous page.
 */
function onPrevPage() {
  if (pageNum <= 1) {
    return;
  }
  pageNum--;
  queueRenderPage(pageNum);
}

document.getElementById("prev").addEventListener("click", onPrevPage);

/**
 * Displays next page.
 */
function onNextPage() {
  if (pageNum >= totalPageCount && current.documentIndex + 1 === pdfDocs.length) {
    return;
  }

  pageNum++;
  queueRenderPage(pageNum);
}

document.getElementById("next").addEventListener("click", onNextPage);

/**
 * @returns PageNumber
 */
  function getPageInfo(num) {
    let totalPageCount = 0;

    for (let i = 0; i < pdfDocs.length; i++) {
      let currentCount = pdfDocs[i].numPages;
      totalPageCount += currentCount;
      if (num <= totalPageCount) {
        return {
          documentIndex: i,
          pageNumber: (currentCount - (totalPageCount - num)),
        };
      }
    }

    return false;
  }

function getTotalPageCount() {
  var totalPageCount = 0;
  for (var docIdx = 0; docIdx < pdfDocs.length; docIdx++) {
    totalPageCount += pdfDocs[docIdx].numPages;
  }
  return totalPageCount;
}

var loadedCount = 0;

function load() {
  // Load PDFs one after another
  pdfjsLib.getDocument(urls[loadedCount])
    .promise.then(function(pdfDoc_) {
    console.log("loaded PDF " + loadedCount);
    pdfDocs.push(pdfDoc_);
    loadedCount++;
    if (loadedCount !== urls.length) {
      return load();
    }

    console.log("Finished loading");
    totalPageCount = getTotalPageCount();
    document.getElementById("page_count").textContent = totalPageCount;

    // Initial/first page rendering
    renderPage(pageNum);
  });
}

<!DOCTYPE html>
<html>
<head>
  <base href="https://mozilla.github.io/pdf.js/"/>
  <meta charset="UTF-8">
  <title>Previous/Next example</title>
</head>
<body onload="load()">

<div>
  <button id="prev">Previous</button>
  <button id="next">Next</button>
  &nbsp; &nbsp;
  <span>Page: <span id="page_num"></span> / <span id="page_count"></span></span>
</div>

<div>
  <canvas id="the-canvas" style="border:1px solid black"></canvas>
</div>

<script src="build/pdf.js"></script>

</body>
</html>

For the sake of not having reliable test documents out there on servers sending a proper CORS-Header, this example simply merges two copies of the default document. If you execute this on your own server, you can of course add any document hosted under the same domain by adding them to the urls array.