I think you need to be able to get a PDF to appear in a dialog. I agree its useful, but I also don't think its something that needs to be added to the product.
I had some success with adding bumbpox to xPages. Bumpbox is a lightbox clone which will handle PDF's. it will only work on desktops though, no mobile for PDF. I can give you some pointers on how to add that to an xPage if you want.
The other option is pdf.js. If you are able to have pdf.js render the pdf on the server you would have a lot more options on how to display the pdf.
The first step I think will be getting the PDF to appear inline. Then work on how to get it to display on mouseover.