PDF Ref Preview

Preview internal links in PDFs on mouse hover.

The script works by following internal links to the target page and location, and render the view in a separate canvas. Some PDFs may not work because the internal links are broken.

This script only works if the viewer is PDF.js.

Firefox

PDF.js is the default viewer in Firefox. To install the script, create a bookmark and add the code below to the location field. See issue #2 for ideas on customizing the script to individual preferences.

javascript:(async function togglePreview() {
  const app = window.PDFViewerApplication;
  const anchor = document.getElementById("viewer");
  if ("_previewHandler" in app) {
    anchor.removeEventListener("mouseover", app._previewHandler);
    delete app._previewHandler;
    delete app._previewing;
    return;
  }

  const box = anchor.getBoundingClientRect();
  const halfWidth = (box.left + box.right) / 2;
  const destinations = await app.pdfDocument.getDestinations();
  app._previewing = false;

  async function mouseoverHandler(event) {
    if (!event.target.offsetParent.hasAttribute("data-internal-link") || app._previewing) return;

    const hash = event.target.hash;
    const parent = event.target.parentElement;

    const preview = document.createElement("canvas");
    const previewStyle = preview.style;
    previewStyle.border = "1px solid black";
    previewStyle.direction = "ltr";
    previewStyle.position = "fixed";
    previewStyle.zIndex = "99";
    previewStyle.top = `${event.clientY + 4}px`;
    previewStyle.boxShadow = "5px 5px 5px black, -5px 5px 5px black";

    const namedDest = decodeURIComponent(hash.substring(1));
    const explicitDest =
      namedDest in destinations
        ? destinations[namedDest]
        : JSON.parse(namedDest);
    const pageNumber = app.pdfLinkService._cachedPageNumber(explicitDest[0]);

    app.pdfDocument.getPage(pageNumber).then(function (page) {
      const tempViewport = page.getViewport({ scale: 1.0 });
      const height = tempViewport.height * 1.2 * app.pdfViewer.currentScale;
      const width = tempViewport.width * 1.2 * app.pdfViewer.currentScale;
      const leftOffset =
        event.clientX > halfWidth ? (2 * width) / 3 : width / 3;
      previewStyle.height = `${height}px`;
      previewStyle.width = `${width}px`;
      previewStyle.left = `${event.clientX - leftOffset - 4}px`;

      let offsetY;
      switch (explicitDest[1].name) {
        case "XYZ":
          offsetY = explicitDest[3];
          break;
        case "FitH":
        case "FitBH":
        case "FitV":
        case "FitBV":
          offsetY = explicitDest[2];
          break;
        default:
          console.log(`Oops, link ${explicitDest[1].name} is not supported.`);
      }

      const scale = 4;
      const viewport = page.getViewport({
        scale: scale,
        offsetY: (offsetY - tempViewport.height) * scale,
      });

      preview.height = viewport.height;
      preview.width = viewport.width;

      const renderContext = {
        canvasContext: preview.getContext("2d"),
        viewport: viewport,
      };
      page.render(renderContext);
    });

    anchor.prepend(preview);
    app._previewing = true;

    parent.addEventListener("mouseleave", function (event) {
      preview.remove();
      app._previewing = false;
    });
  }
  anchor.addEventListener("mouseover", mouseoverHandler);
  app._previewHandler = mouseoverHandler;
})();

Chrome

To use in Chrome, follow the build and install instructions src/pdf.js README file for Chrome, e.g.,

git clone git@github.com:belinghy/PDFRefPreview.git --recu
cd PDFRefPreview/src/pdf.js

npm install -g gulp-cli
npm install
gulp chromium

Then open Chrome, go to Tools > Extension and load the (unpackaged) extension from the directory build/chromium.

The preview function can be customized by tweaking src/pdf.js/web/pdf_ref_preview.js. The togglePreview function is called in src/pdf.js/web/app.js.

By default, the preview function is enabled on every PDF and can be toggled by pressing q.