I think you have the wrong model for apps. Doing one thing is almost always having the full document CRUD loop.
Once you have a pdf viewer users want rotating pages, then bookmarks, then highlights, then form filling, etc.
That “just a pdf reader” is strictly worse than every pdf editor (which have to meet the minimum requirement of a pdf reader).
Apple’s Automator and Preview combo for me are indispensible and perfectly follow the Unix ideal. Automator composes well, and Preview does all the pdf editing well, and together they’re excellent software.
The version of Preview Apple just released on iPad is however basically just a reader, and do it hasn’t made itself relevant to any of my workflows or needs.
Full featured plus composable. That’s the sweet spot for the best apps.
Side note: I’m surprised there’s no ffmeg or imagemagik for pdfs (maybe there is?). Someone should build that.