Two years later, I may have found something. There is now an app on the Mac App Store called PDF Reader X. It's free, has tabs and supports annotation!
PDF files are popular precisely because the formatting is fixed. A PDF author can be sure that their document looks the same on any device that displays it. So I'm afraid you're asking for a feature that goes against one of the basic premises of the format.
That said, some PDFs have a text-only layer that can be displayed directly. On an iPhone, I'd recommend Goodreader for this. It will display the formatted PDF, then you can click to display the text-only version. That version may be bigger and more legible for you.
Having a text layer depends on the author of the PDF, but I would think Goodreader may be a step in the right direction for you.
Best Answer
I think the DocuWorks viewer will, but does not seem to be available in the U.S. store right now for some reason.