Reviewing a document is a different job from editing one, and the tools should not feel the same. When you review, you are not changing what the document says — you are leaving marks for a person who will read it after you, and the words underneath need to survive exactly as they are so the author can see what you meant.
So everything here sits on top. A highlight is a coloured layer over the words, an underline is a line beneath them, a note is a marker anchored to a place. All of them are real PDF annotations, which is why the person you send the file to sees them in whatever software they happen to open it in, rather than getting a flattened image with your comments baked in.
The part usually done badly is tool selection. The conventional approach is a strip of eleven buttons — highlight, underline, strikeout, squiggle, note, text, shapes, arrow, and so on — where you arm one and then hunt for the words. Here you select the text first, and then choose what the mark means. Highlight, underline and strike through are one control with three treatments, because they are three meanings for the same gesture rather than three separate tools.
Comments get a list. Twelve remarks scattered across nine pages is unreadable if the only way to find them is scrolling, so they appear beside the document in page order, and replies thread under the note they answer.
One boundary is held deliberately. You can draw a filled box over something, and it will always be called covering — never redaction. The text stays in the file underneath and can be recovered by anyone who selects it. When that matters, the tool that permanently deletes text from the file is one click away, and this page will keep pointing at it rather than letting a black rectangle imply something it cannot deliver.
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