Since web browsers don't support displaying PDFs within themselves and require an interpreter (like PDF.js) it takes a little extra effort to get this working. For PDF.js, which seems like the best way to show a PDF on a webpage, it looks like it is possible to use it offline:
The path you would use is ${resourcesFolderName}/filename.pdf or you may need to hardcode it to something like index.hyperesources/filename.pdf if placed inside an HTML widget.
So once you have that working, you will likely want to setup a Kiosk situation where the iPad can download all files locally and display them without internet access. Here's a few tools for that: Displaying a Tumult Hype document in a Kiosk or as an app
If the PDFs are just a few pages, you may save yourself a lot of trouble and either export them to HTML or export them to a sequence of images that you create a slideshow with.