Changing the Font Size or Color of Page Text
Unfortunately the nature of PDF files makes this impossible.
PDF files represent a document with a fixed-layout, unlike in HTML/ePub or Word documents there is no notion of text flow, glyphs are laid out one by one. Increasing the size of individual glyphs would just make them overlap each other, or if their position is adjusted as well, the result would resemble page zoom, not changing the font size.
PSPDFKit for iOS supports rendering HTML to PDF which you can then view and annotate in PSPDFKit.