Annotation is a layer model — use it like one
Everything this tool adds sits on top of the existing page, which is exactly the right mental model for the tasks it serves: filling a value onto a flat form that has no fields, marking a correction on a draft, highlighting the clause that matters, circling the number that’s wrong. The page underneath — vector text, embedded images, fonts — is copied into the output untouched, so nothing loses sharpness and the original text stays selectable beneath your marks.
That layer model also tells you when to reach for a different tool. If the form has real, clickable fields, the PDF Form Filler writes into them properly instead of painting text on top. If the mark you need is a signature, the Sign PDF tool handles drawn and typed signatures with placement built for that job. And if your goal is to hide content rather than add it, a highlight or a drawn box is the wrong instrument entirely — an opaque rectangle over text leaves the text present and extractable underneath. Use Redact PDF, which removes the content itself.
Why flattening on save is the safe default
This tool bakes your marks into the page content at save time rather than storing them as comment objects, and that choice is deliberate. Comment-style annotations depend on viewer support: they can be hidden with one click, silently dropped by some mobile viewers and print pipelines, and stripped by document-processing systems. For the use cases above — a form value, a permanent note, a correction — you want the mark to be as durable as the document. Flattened content is exactly that: it renders everywhere the page renders, prints every time, and can’t be peeled off without editing the page itself.