The best way to include a downloadable PDF is using the Viewer Display options (available with in5 Pro plans and higher).
You can either include a custom PDF (e.g., if you've created a separate file specifically for print) or in5 can automatically generate one for you from the current document.
See how it works in the video below.
If you want to link to separate "resource" PDFs from your main document, you can attach a local folder using the Resources section of the in5 dialog.
You can then link to the files in that folder as relative hyperlinks.
In the output, your attached folder will have a path of assets/resources/ so a file named mypdf.pdf can be referenced and used as a Hyperlink (using the Hyperlinks panel or a Go to URL button action in the Buttons & Forms panel) with the following path: assets/resources/mypdf.pdf
For best viewing of lightbox (non-downloaded PDFs) on mobile, see this article:
https://ajarproductions.com/blog/2017/07/03/how-to-load-a-pdf-in-a-lightbox-on-mobile-safari/