I'm reading Twitter on an iPad Air. Twitter links to an article that has a downloadable PDF. I click to download. It's large. The selection to open in iBooks flashes by, so I can't choose. But the PDF opens on the Air. Am I viewing this PDF in Air memory, or is the physical PDF stored on the Air? Where if so? I went back and re linked to the PDF, this time I was fast enough to click open in iBooks. So I finally have a physical file in iBooks. But is the first attempt still on the Air some where?