I'm hoping to avoid the behavior in which a PDF is download to a mobile device

This is not something I've seen occur, on iOS anyway. It always displays in the browser. It does give the user the option to save to iBooks(?) but you'd assume if they click that they would be doing so with an understanding of what they are doing...

You might be able to do something clever to change the link to a PDF based on screen size so that you could have a different one when accessed via a small screen - but I have to say if you don't want the user to download it and you want it to be responsive... is PDF really the right solution? Why do you want to display the content as PDF in the first place? Could you not port the content into Adapt?

If it's the 'page flip' effect you really want, could you use something like flipping book or pageflip (or flexpaper as Sue suggests) and link to the output of that?