From the screenshot it looks like the font used to display the text is not the font in the document being quoted but Courier or some other fixed width font. This is a poor choice from a readability standpoint. Which suggest another option to work on, besides variable text size - the ability for a user to choose the font they prefer for reading. Personally I like Verdana Regular, which is a medium weight sans serif font with excellent letter spacing. In print serif fonts are generally easier to read, but on screen sans serif fonts work better. In any case, if this app gave users a choice, they could pick the typeface that works best for them. A monospaced and/or fixed width font is the absolute worst choice, and suggests the author, though well intentioned, is not well informed.
I think I'll wait to see how this app develops.