Yer_Man wrote: I wonder if you're confusing Preview…
Now, in Files on the iPhone, pdfs and pictures are shown with an iOS version of Preview. As stevenlt says, it seems pretty awkward. I hadn't discovered "Open with," and, as 6x6 said, that lets you use QuickLook, so it's smoother. Looking at pictures in iCloud is weird, anyway, since, unlike Photos, there are no preview versions to speed up scanning. You get the thumbnail and a "downloading" message. I assume it's download a preview version, but I'm not sure.
However, I just tried changing to Open with Preview to see the difference, and I can't figure out how to get back to QuickLook! Oh, OK, QuickLook appears in a different spot in the long press menu.