Changing font size
Unfortunately, the nature of PDF files makes it so that changing the font size or color of page text is impossible.
PDF files represent a document with a fixed layout, and unlike in HTML, EPUB, or Word documents, there is no notion of text flow. Instead, glyphs are laid out one by one. Increasing the size of individual glyphs would just make them overlap, and if their position were to be adjusted as well, the result would resemble page zooming and not changing the font size.