PDF Thumbnails

This plugin hooks into the media manager and generates a thumbnail everytime a
PDF is uploaded. Generated thumbnail is an image of the first page in the
uploaded document and is named PDFNAME-thumbnail, where PDFNAME is replaced
by uploaded document filename.

Generated thumbnails are equivalent to featured
images so that common thumbnail
functions like get_post_thumbnail_id() can be used for PDF attachments. See
Post Thumbnails for information
on how you can use thumbnails efficiently.

Integration with the javascript media API is not yet implemented, therefore, you
may need to reload the page before you can see generated thumbnail after an
upload.

Shortcodes

It is possible to display a thumbnail linking to an uploaded PDF using the [pdf_thumbnails_link]
shortcode. The following attributes are supported:

• id – Attachment ID (required)
• size – Thumbnail size (optional)
• title – Anchor title attribute (optional)
• target – Anchor target attribute (optional)
• download – Anchor download attribute (optional)

Example 1: Display link to PDF with ID = 172 using default thumbnail size

[pdf_thumbnails_link id="172"]

Example 2: Display link to PDF with ID = 172 using thumbnail size (default 150×150)

[pdf_thumbnails_link id="172" size="thumbnail"]

Thanks to mirgcire for providing the first
version of the [pdf_thumbnails_link] shortcode.

Developer API

In most cases it should be sufficient to use built-in thumbnail functions from
the WordPress API (get_post_thumbnail and similar). If you need to modify the
way thumbnails are generated, you can override image generation with
the pdf_thumbnails_generate_image_blob filter.

Example 1: Increase resolution for all generated PDF thumbnails

// $blob is the current image blob (defaults to null, can be used for chaining)
// $filename is the PDF filename
add_action('pdf_thumbnails_generate_image_blob', function ($blob, $filename) {
    $imagick = new Imagick();
    $imagick->setResolution(200,200);
    $imagick->readImage($filename);
    $imagick->setIteratorIndex(0);
    $imagick->setImageFormat('jpg');
    return $imagick->getImageBlob();
}, 10, 2);

It is possible to modify generated thumbnail links using the pdf_thumbnails_link_shortcode
filter. The following attributes are available:

• $html – Generated HTML code to be displayed
• $attachmentId – Sanitized ID of the PDF attachment
• $size – Sanitized thumbnail size
• $atts – Shortcode attributes (not sanitized)
• $content – Shortcode content (not sanitized)

Example 2: Wrap thumbnail link in figure and append caption

add_filter('pdf_thumbnails_link_shortcode', function ($html, $attachmentId, $size, $atts, $content) {
    return "<figure>$html <caption>Click to open image $attachmentId</caption></figure>";
}, 10, 5);

TODO

Add generated image to media browser after upload.

Outline of an implementation based on the javascript media API:

// New uploads
wp.Uploader.queue.on('add', function (attachment) {

    if (attachment.subtype !== 'pdf') {
        return;
    }

    findThumbnailFor(attachment.ID).then(function (data) {

        // Add attachment thumbnail to browser
        var attachment = wp.media.model.Attachment.get(id)
        attachment.fetch().done(function () {
            wp.media.editor.open().state().get('library').add(generated attachment)
        });

    });
});

Filter: ajax_query_attachments_args