Workflow 9 min read 14 October 2026

PDF Annotation Guide: Every Type, Tool, and Workflow Explained

PDF annotations let you highlight, comment, draw, stamp, and measure — all without altering the underlying page content. This guide covers every annotation type, how they are stored in the PDF specification, which tools support them, and how to manage annotation workflows for review and collaboration.

PDF annotations are a layer of content that sits on top of a PDF page without modifying the underlying text or images. They include highlights, sticky notes, freehand drawings, stamps, measurement tools, file attachments, and clickable links — all stored as separate objects that can be shown, hidden, filtered, or removed without touching the original page content. The PDF specification defines over 20 annotation types across markup, widget (form fields), media, and navigation categories. Understanding which annotation types exist, how they are stored, and which tools handle them reliably is the foundation of any document review or collaboration workflow. This guide covers every major annotation type, the tools best suited to each, how annotations are stored in the PDF file, how to manage annotation workflows across teams, and how to flatten annotations when you need a finalised, non-editable output.

Annotation types: the complete reference

The PDF specification (ISO 32000) defines annotation subtypes via the `/Subtype` key in each annotation dictionary. The most important types: **Text markup annotations** — Applied to specific text passages using QuadPoints arrays that define the precise character positions: - `/Highlight` — Yellow (or coloured) background behind text. The most widely used annotation type. - `/Underline` — Line beneath text. Used for editorial markup. - `/StrikeOut` — Line through text. Indicates text to be deleted. - `/Squiggly` — Wavy underline. Used for spell-check or error indicators. **Note annotation (/Text)** — A sticky note pop-up at a specific page point. The note collapses to a small icon; clicking opens a comment pop-up. The primary type for reviewer comments. **Free text annotation (/FreeText)** — A text box with a visible border, positioned anywhere on the page. Unlike sticky notes, the text is always visible without clicking. **Ink annotation (/Ink)** — A freehand drawn path. Used for handwriting, sketching, or circling content on a page. **Stamp annotation (/Stamp)** — A rubber-stamp appearance drawn on the page. Standard stamps include "APPROVED", "DRAFT", "CONFIDENTIAL", "FOR COMMENT", "FINAL". Custom stamps (company logos, reviewer IDs) are also supported. **Link annotation (/Link)** — A clickable rectangle that triggers a URI action (web link) or GoTo action (internal navigation). See URI Annotation. **Widget annotation (/Widget)** — Interactive form field. Part of the AcroForm layer. See AcroForm. **File attachment annotation (/FileAttachment)** — Embeds a file at a page location. Appears as a paperclip icon; clicking opens the attached file. **Redact annotation (/Redact)** — A two-stage annotation: first marks content for redaction; then the Apply operation permanently removes the marked content. See Redaction. **Measurement annotations** — Distance, perimeter, and area measurements used in engineering and architectural PDFs. Store numerical measurement data alongside the visual mark.

How annotations are stored in the PDF structure

Annotations in a PDF are stored in the page dictionary's `/Annots` array — a list of annotation dictionaries (or indirect references to them). Each annotation dictionary contains at minimum: - `/Type /Annot` — Identifies it as an annotation - `/Subtype` — The annotation type (Highlight, Text, Ink, etc.) - `/Rect` — A four-number bounding rectangle `[x1 y1 x2 y2]` in PDF coordinates - `/Contents` — The text content (comment text for sticky notes, URL for links) - `/AP` — Appearance stream dictionary (how to draw the annotation visually) - `/P` — Indirect reference to the parent page Text markup annotations additionally use: - `/QuadPoints` — An array of 8n numbers describing the exact character rectangles covered by the markup. Eight numbers per quadrilateral: lower-left, lower-right, upper-right, upper-left (in PDF coordinates). The separation between page content (the `/Contents` page stream) and annotation data (the `/Annots` array) is fundamental: it means annotations can be added, removed, or modified without touching the original page content. This is the PDF overlay model described in the PDF overlay glossary entry.

Annotation review workflows: tools and platforms

**Adobe Acrobat** — The most complete annotation toolset. Acrobat's Comment & Markup toolbar covers all PDF annotation types. The Comment List panel shows all annotations in a sortable, filterable list. Acrobat supports: - Creating, editing, and deleting all annotation types - Annotation replies (threaded comments) - Setting annotation status: Accepted, Rejected, Cancelled, Completed, None - Exporting annotations to FDF/XFDF for import into other applications - Summarising comments as a PDF or Word document **Adobe Acrobat Reader (free)** — Can view and add sticky notes, highlights, and basic text annotations. Cannot create stamps or ink annotations. Sufficient for light review. **Foxit PDF Reader/Editor** — Foxit provides comprehensive annotation tools including stamps, drawing tools, and measurement annotations. Strong multi-user review with shared review capability. **Browser-based annotation** — Chrome, Firefox, and Edge all have built-in PDF viewers that support basic highlighting and sticky notes. Annotations are saved back into the PDF file. **iPad annotation apps** — Notability, GoodNotes, and PDF Expert are widely used for handwritten annotations with Apple Pencil. Export annotated PDFs with ink annotations embedded. **Collaborative annotation** — Tools like Hypothesis (open-source), Perusall (education), and Acrobat's Shared Review feature support multi-user annotation workflows where multiple reviewers can annotate simultaneously and see each other's marks.

Flattening annotations: when and how

Flattening merges all annotations into the underlying page content, producing a static PDF where the annotations are permanently visible but no longer interactive or removable. This is the right step before: - Sending a final approved document (prevents future editing of comments) - Printing where annotations must appear in the printed output - Archiving as PDF/A (annotations must be flattened or have correct appearance streams for PDF/A conformance) - Redaction — after placing Redact annotations, Apply Redactions permanently removes the marked content and flattens the redact annotation visually **Tools for flattening:** Adobe Acrobat Pro: Tools > Print Production > Flatten Command-line with Ghostscript: ```bash gs -dNOPAUSE -sDEVICE=pdfwrite -dFlattenAnnotations -sOUTPUTFILE=flat.pdf -dBATCH input.pdf ``` Python with pypdf: ```python # pypdf doesn't flatten directly; use Ghostscript or pdf-lib for true flattening # Alternatively, render each page to image and reassemble (lossy approach) ``` Our Annotate PDF tool lets you place annotations in the browser and download the result — annotations are embedded in the file so they appear in any PDF viewer.

PDF annotations are one of the format's most practical features — enabling collaboration, review, and markup without ever modifying the underlying document. The key decisions in any annotation workflow are: which tool to use (Acrobat for comprehensive control, browser-based for zero friction), whether to use sticky notes (collapsible, multi-threaded) or free text (always visible), and when to flatten (finalisation, print, archive). Understanding that annotations are stored separately from page content and can always be removed explains much of what makes PDF-based review reliable and reversible.

Frequently asked questions

Do PDF annotations change the underlying document content?

No — annotations are stored in a separate `/Annots` array in the page dictionary, not in the page content stream. The underlying text, images, and layout are completely unchanged. Annotations can be added, modified, or deleted without affecting the original page content. Only flattening (merging annotations into the content stream) makes them permanent. This is why you can highlight text, then remove the highlight, with the original text intact.

How do I remove all annotations from a PDF?

In Adobe Acrobat Pro: open the Comments panel, select all (Ctrl+A), delete. Or use Edit > PDF > Edit > Select All (for objects) and then filter to annotations. Command-line: `qpdf --remove-annotations input.pdf output.pdf` (QPDF 11+). Python: `pikepdf.open("in.pdf"); del page["/Annots"]` for each page. Some online tools also offer annotation stripping — but check they process files locally if the PDF contains sensitive content.

Can I export PDF annotations to Word or Excel?

Adobe Acrobat can export annotations as a summary PDF or a Word document (Comments > Export All to Data File > XFDF, then import or summarise). For Excel/CSV, use the command-line tool pdfannots (Python) which extracts annotations including highlighted text, sticky notes, and comments to a structured text or JSON format suitable for import into spreadsheets.

Will annotations created in Acrobat be visible in other PDF viewers?

Standard annotation types (Highlight, Text/sticky note, Stamp, Ink, FreeText) defined in ISO 32000 should be visible in any conforming PDF viewer. Appearance streams (/AP dictionaries) determine how they render — well-formed annotations have appearance streams that any viewer can render. Proprietary Acrobat features (annotation replies stored in private dictionaries, some stamp types) may not render correctly in non-Adobe viewers. Browser PDF viewers support the most common types; older mobile PDF apps may skip annotation rendering entirely.

Try it yourself

Everything in this article can be done directly in the free PDFTheGuide editor — no account required.

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