Annotation

The PDF standard defines many annotation types: text markup (highlight, underline, strikeout, squiggly), notes, free-hand ink, shapes, stamps, links and more. Viewers can show, hide or remove them independently of the page.

Form field widgets are technically annotations too, which is why forms and markup often share the same machinery.

Annotations are stored per page, separately from the page’s content, and each one keeps its own position, type and properties such as colour, author and date. Most also carry an appearance, so a viewer can draw them the same way even if it doesn’t fully understand the type.

This separation is what makes review work. A reviewer’s comments can be listed, replied to, hidden or deleted, and the document underneath stays exactly as it was. It also means a highlight doesn’t change the text it covers, and removing an annotation doesn’t damage the page.

When a document is finished, annotations are often flattened into the page so they look the same everywhere and can’t be removed. Whether to do that depends on the workflow: keep them while people are still reviewing, and flatten them when the file goes out.

For an app, the practical choice is who may do what. A reader might only add highlights, a reviewer might add and delete comments, and a final copy might allow neither, so it helps when the editor lets you choose which tools appear.