What Are PDF Annotations? Comments, Highlights, and Stamps Explained

Annotation Types

• Text annotations: the "sticky note" — a small icon that expands to a pop-up comment bubble when clicked
• Markup annotations: Highlight (yellow background), Underline, Strikethrough, Squiggly — mark text with a colored overlay; the marked text content is referenced in the annotation
• Ink annotation: freehand drawing paths, like a digital pen over the page
• Stamp annotation: a named graphic stamp (Approved, Confidential, Draft, Received) or custom image placed on the page
• Link annotation: a clickable rectangle that triggers an action — navigate to another page, open a URL, submit form data
• File attachment annotation: an embedded file attached to a specific location on the page (paperclip icon)
• Widget annotation: form fields are implemented as widget annotations — text boxes, checkboxes, radio buttons, list boxes
• Redact annotation: marks an area for redaction (before the redaction is applied)

How Annotations Are Stored

Annotations are stored as an array of annotation dictionaries in the page's /Annots entry. Each annotation dictionary specifies: the annotation type (/Subtype), its rectangle (/Rect), its appearance (either a referenced appearance stream in /AP or derived from standard appearance rules), the author (/T), the creation date (/CreationDate), and content (/Contents for text annotations). Annotations are separate from the page content stream, which is why they can be shown/hidden independently.

Annotations and Printing

By default, most annotations print when you print a PDF. Each annotation has a /F (flags) field with a Print bit — when set, the annotation appears in print output. Some annotation types (like sticky notes) have the Print flag unset by default in some applications, so they appear on screen but not in print. To print annotations as they appear on screen: in Acrobat's Print dialog, set "Document and Markups" or "Document and Stamps." To print without any annotations: set "Document Only."

Annotation Review Workflows

PDF's annotation system was designed for document review workflows: multiple reviewers can add comments to a document, and their annotations are collected into a Comments panel with author, date, and reply threads. Adobe Acrobat supports comment import/export (FDF and XFDF formats) for collecting reviews from multiple parties. The "Send for Review" feature in Acrobat creates a managed review workflow. For collaborative review without Acrobat, services like Adobe Document Cloud and various third-party tools provide annotation workflows over standard PDFs.