Think of a PDF page as a glass panel. The page content — text, images, graphics — is painted on the glass itself. Annotations are sticky objects placed on top of the glass: a yellow highlighter strip that can be peeled off, a pink sticky note that can be moved, an "APPROVED" stamp that can be deleted. They overlay the content without touching it. That's why a PDF reviewer can add 50 comments and none of them change a single character of the original document. Annotations are stored in the PDF's /Annots array, are governed by annotation flags, and each carries an appearance stream that defines how it looks on screen and in print.
What Are PDF Annotations?
In the PDF specification, an annotation is an interactive object associated with a location on a PDF page. Annotations are stored in the page dictionary's /Annots array as indirect references to annotation dictionaries. Each annotation dictionary contains at minimum:
- /Type /Annot — identifies this dictionary as an annotation
- /Subtype — specifies the annotation type (Text, Highlight, Stamp, Link, Widget, etc.)
- /Rect — the bounding rectangle on the page in user space coordinates
- /F — annotation flags controlling visibility, print behaviour, and locking
- /AP — appearance stream dictionary defining how the annotation looks
Markup annotations (used for document review) additionally carry /T (author/title), /Contents (comment text), /CreationDate, /M (modification date), and an optional /Popup reference to the floating comment bubble.
Annotations vs. page content: Annotations never touch the page content stream. A highlighted word still exists as original text in the content stream — the highlight is a semi-transparent yellow rectangle annotation placed on top. This is why you can delete all annotations and the original document is completely unaffected.
All PDF Annotation Subtypes
| Subtype | Description | Common Use |
|---|---|---|
| Text | Sticky note — collapsed icon with popup comment | Reviewer comments on document content |
| Highlight | Semi-transparent colour band over text | Mark important passages for review |
| Underline | Line drawn under selected text | Editorial markup and emphasis |
| StrikeOut | Line through selected text | Mark text for deletion in review |
| Squiggly | Wavy line under text (spell-check style) | Flag spelling errors or uncertain content |
| FreeText | Text box placed anywhere on page | Labels, callouts, inline comments |
| Stamp | Pre-designed rubber stamp graphic | APPROVED, DRAFT, CONFIDENTIAL, VOID markings |
| Ink | Freehand path drawn on the page | Handwritten signatures, freehand drawings |
| Line | Straight line with optional arrowheads | Callout lines, measurement annotations |
| Square / Circle | Rectangle or ellipse shape | Circle feedback areas, bounding box markup |
| Link | Clickable region with GoTo or URI action | Hyperlinks, cross-references, navigation |
| Widget | Interactive form field (text, checkbox, radio, button) | AcroForm interactive forms |
| FileAttachment | Embedded file with paperclip icon | Attach supporting files to specific pages |
| Caret | Insertion marker (^) between characters | Mark text insertion point in editorial review |
Real-World Examples
Multi-Reviewer Document Review: 4 Editors, One PDF
A publishing house sends a manuscript to four editors. Each editor adds Highlight annotations in their chosen colour (one per reviewer), Text sticky notes with comments, Strikeout annotations on sections they recommend cutting, and Caret annotations marking insertion points for new content. The document is circulated as a single PDF. In Acrobat's Comments pane, annotations are sorted by page and author — each editor's markup is filterable by name and colour. The original manuscript text is unchanged; all feedback layers on top of it. The author receives one file with 160 annotations from all four reviewers.
Contract Review: Stamp + Comment Workflow
A corporate legal team reviews a supplier contract. Sections that are approved receive a green "APPROVED" Stamp annotation. Sections requiring revision receive a red "REVISE" Stamp plus a linked Text popup explaining the required change. Sensitive clauses are circled with Circle annotations with a comment specifying the concern. When the final version is agreed, all annotations are checked, the Stamp is changed to "EXECUTED," and the annotations are flattened — merging them permanently into the page content before the signed PDF is archived.
Technical Drawing Review with FileAttachment Annotations
A structural engineer reviews an architectural PDF drawing. They add FreeText callout annotations pointing to specific elements with technical notes, circle questionable details with Circle annotations, and attach a calculation spreadsheet as a FileAttachment annotation pinned to the relevant structural section. The reviewing engineer can open the spreadsheet directly from within the PDF by clicking the paperclip icon — all feedback and supporting data stays in one file, eliminating version confusion across email chains.
Why PDF Annotations Matter
Non-Destructive Markup
Annotations never modify the original content stream. Review, comment, and mark up documents without the risk of altering the underlying text, images, or layout.
Collaborative Review
Multiple reviewers can annotate the same PDF, each with their own author name and colour. All comments are consolidated in one file — no merging separate Word documents.
Audit Trail
Every annotation records the author, creation timestamp, and modification timestamp — providing an immutable audit trail of who reviewed the document and when comments were made.
Print Control
Annotation flags (Print/NoView) let you control which annotations appear in print output — review comments can print with the document or be suppressed for clean distribution copies.
Workflow Automation
Annotations can be extracted programmatically — allowing DMS systems, OCR pipelines, and review workflows to read, sort, and act on comment data without human intervention.
Flatten for Archiving
Annotations can be flattened into the content stream for archiving or distribution — converting interactive review markup to permanent, viewer-independent page content.
Annotation Dictionary Structure
<< /Type /Annot /Subtype /Highlight /Rect [72 692 310 708] % bounding box (x1 y1 x2 y2) /QuadPoints [72 708 310 708 72 692 310 692] % exact quad coverage /C [1 1 0] % colour (RGB yellow) /T (Alice Reviewer) % author /Contents (Check this clause) % comment text /CreationDate (D:20260312143022Z) /F 4 % Print flag >>
Common Mistakes to Avoid
- Not flattening annotations before archiving or distributing final documents. Unflattened annotations remain editable and can be deleted by anyone with a PDF editor. For final contracts, approved drawings, or archived documents, flatten annotations to embed them permanently in the content stream.
- Confusing annotations with page content edits. Adding a FreeText annotation does not add text to the underlying content stream — it adds an overlay. If the PDF is processed by a tool that strips annotations, the FreeText text disappears. Use true content editing for permanent text changes.
- Creating annotations without appearance streams. Annotations without an /AP (appearance stream) dictionary rely on the viewer to render them — different viewers may render them differently or not at all. Always ensure annotations have defined appearance streams, especially for Stamp and Ink annotations.
- Not setting the Print flag for annotations that must print. Annotations without the Print flag (/F bit 3) do not appear in printed output. If review comments or stamps must print, verify the Print flag is set. PDF/A additionally requires the Print flag on all annotations.
- Expecting annotations to survive all PDF processing. Compression, optimisation, linearisation, and some conversion tools may strip or flatten annotations as a side effect. Always keep a version of the PDF with annotations intact before processing for distribution or archiving.
Frequently Asked Questions
PDF annotations are interactive overlay objects stored in a page's /Annots array that sit on top of the page content without modifying it. Types include sticky notes (Text), Highlight, Underline, Stamp, FreeText, Ink, Link, Widget (form fields), FileAttachment, and more.
Page content (text, images, graphics) is in the page's content stream — permanent and unmodifiable without editing the stream. Annotations are stored separately in /Annots — they overlay the content and can be added, changed, or deleted without touching the original. Deleting all annotations leaves the document completely unchanged.
Markup annotations are review-focused annotations that carry an author name, creation date, and optional popup comment. Types include: Text, FreeText, Line, Square, Circle, Polygon, Highlight, Underline, Squiggly, StrikeOut, Stamp, Caret, Ink, and FileAttachment.
Each annotation is a PDF dictionary listed in the page's /Annots array. It contains the subtype (/Subtype), position (/Rect), flags (/F), appearance stream (/AP), colour (/C), author (/T), comment text (/Contents), and creation/modification dates. Appearance streams define the visual rendering in a self-contained content stream.
Yes. Flattening merges annotation appearance streams into the page content stream — making them permanent, non-editable page content that renders identically in all viewers. Use flattening to finalise reviewed documents, make stamps permanent, and prepare PDFs for archiving.
Yes. PDF/A permits Text, Highlight, Underline, Link, Stamp, and Widget annotations — but requires the Print flag on all of them and an /AP appearance stream defined. Movie and Screen multimedia annotations are not permitted in PDF/A-1/2. Annotations without /AP are non-conformant.
Annotate & Review PDFs — Free
Add highlights, comments, stamps, and markup to any PDF directly in your browser with PDFlyst.
Open PDF Editor — Free