Adding a line to a PDF and changing a line that is already there are two different operations, even though on screen they look like the same thing. One draws something new. The other has to reach into what is already drawn — a set of instructions with their own font, position and spacing, where the font is often embedded as a partial copy carrying only the characters that page used. So my rule is short: change what you changed, leave the rest alone. The edit aims at the smallest thing that has to move, and the result is checked against the original before anything is kept — if the page moved in a way it should not have, the edit does not happen at all. Longer version in Guides: "How to edit existing text in a PDF". ✎