What is PDF annotation?
PDF annotation is the process of adding comments, notes, or other information to a PDF document, making it more interactive and useful.
The latest version of the PDF standard, PDF 2.0, includes 28 types of annotations that can be used to enrich the document, including popular options such as highlighting, text notes, comments, stamps, and signature fields.
PDF annotations can also include file attachments, sound, and interactive 3D content, providing a rich set of features that allow users to work with the document in new ways. These annotations can be added using various software tools and are typically saved within the PDF file itself.
To learn more about the technical details of PDF annotation, you can refer to clause 12.5 of ISO 32000-2:2020 , the official PDF specification.
The main types of annotations
Annotations are objects attached to a page rather than part of its content. This is why they can be edited or removed without changing the underlying document:
- Markup, such as highlights, underlines, strike-outs, and sticky notes.
- Link annotations, which make a region clickable and carry an action.
- Widget annotations, which are the visible part of a form field.
- Stamps, including signature appearances and free text.
- File attachment annotations, which anchor an embedded file to a point on a page.
- Redaction annotations, which mark content for removal before redaction is applied.
Annotations and the page underneath
Because an annotation floats above the page, a highlight does not change the text it covers, and a black box drawn over a passage hides nothing: the text is still there, still selectable, and still extractable. That is the difference between covering and redacting, and it is a recurring cause of accidental disclosure.
Annotations can be exported on their own as XFDF, which is how comments from several reviewers are merged back into one document. Flattening draws them into the page permanently, which is usually what is needed before archiving or sending a final copy.