Opening a PDF in the browser requires a plugin. Since most browsers have PDF plugins nowadays, this is fact can be missed.
Each reader works a bit differently, and there is a known issue with the PDF reader on the iPad where it will only show the first page of a multi-page PDF (and won't scroll down) when loaded into a lightbox (pop-up).
The best workaround is probably to use the trick on the following post to have the Lightbox link load a PDF through Google Docs:
https://ajarproductions.com/blog/2017/07/03/how-to-load-a-pdf-in-a-lightbox-on-mobile-safari/