Actually, I was wrong.
Or we were both kinda right. I had a feeling I was somewhat wrong above, even as I wrote it. iOS 6 did add Share Sheets, you're right, and it was more than I was initially thinking. However, they were/are limited in a number of ways. One of the biggest changes is that they added shared storage (and a data exchange API), so the apps can now do it without creating their own copy of the document (which is obviously troublesome for a storage management perspective, and prevents you from using the same document with many different applications). That is now solved.
And, now developers can introduce their own share sheet extensions, and the end user can re-order applications in the sharing menu (and hide those they don't use).
So, conceivably, you could display the PDF right in-line, with only the one copy (just using that as an example).