You can stack iframe's, Tony Readhead has a tutorial on this.
In fact I would say check out his website: LINK
This said what I would do is set a logic block so if OS = iOS then open a new page, so not using an iframe.
Why I would do this is because at some point stacking frames may well brake as its bit of a workaround.
The ht_url component already has this set, just add the component and change the icons.
Then in the Skin Configuration button set to open in player.
This will open the PDF in the player but open a new window/tab for iOS.