Can I change the text that is already in my PDF?
No, and any tool that says it can is showing you something else. A PDF records where each glyph sits on the page, not the sentence it belongs to, so there is no paragraph to re-flow — and a line set in a font that was embedded as a subset cannot be typed into with that same font, because the glyphs you would need were never included. This tool adds to the page: cover what is wrong and put the right thing beside it.