Welcome on these forums 🙂
To show a PDF in a light box, you can do the following:
1. Enable the PDF option on Settings > Media, save and make sure the Autodetect option in the new PDF section is checked.
2. Edit your page/post and create a link to your PDF file. If you uploaded the PDF to your WordPress library, just insert it like you would insert an image. This will create a link to the file and if the auto-detect from Easy FancyBox is working like it should, the link (when clicked) will automatically open in a light box.
If this does not work like expected, please send me a link to the page where you are trying to make it work. I may be able to help you further with more specific pointers 🙂