Plugin:
Tutorial:
You can easily open a PDF file in a lightbox popup using the Wonder Lightbox plugin. By default, the PDF file will open with the "Automatic Zoom" option enabled.
To define a different zoom option, add an additional parameter to the end of the PDF file URL. The available options are: #zoom=page-width, #zoom=page-height, and #zoom=page-fit.
To make the PDF page fit the entire lightbox width, you can add #zoom=page-width to the end of the PDF URL. For example:
https://www.wonderplugin.com/wp-content/uploads/2017/02/how-to-create-a-wordpress-post-carousel.pdf#zoom=page-width
The lightbox code will be as follows:
<a href="https://www.wonderplugin.com/wp-content/uploads/2017/02/how-to-create-a-wordpress-post-carousel.pdf#zoom=page-width" class="wplightbox" data-barheight="0" data-width="1200" data-height="1200" data-showsocial="false">Open PDF in Lightbox</a>
An online demo is as follows: Open PDF in Lightbox