How to Annotate a PDF: Comments, Highlights, and Markups

What Is PDF Annotation?

PDF annotation is the practice of adding marks, notes, and comments to a PDF without changing the underlying document. Annotations sit in a separate layer on top of the content — the original text and images remain intact, and annotations can be shown, hidden, or removed independently.

Annotation is essential for document review workflows: legal teams mark up contracts, editors comment on manuscripts, engineers redline drawings, and students highlight study materials.

Types of PDF Annotations

Text Annotations

• Highlight — Colour background on selected text (like a physical highlighter)
• Underline — Line beneath selected text for emphasis
• Strikethrough — Line through text to mark deletions
• Squiggly underline — Wavy line to mark errors

Note/Comment Annotations

• Sticky note — Pop-up comment you click to read; used for questions or suggestions
• Text box — Visible text placed directly on the page
• Callout — Text box with a pointer arrow aimed at specific content

Drawing Annotations

• Pencil/Freehand — Free-form drawing using mouse or stylus
• Line, Arrow — Straight connector lines pointing to elements
• Rectangle, Oval — Boxes drawn around content to highlight areas

Stamp Annotations

Standard stamps: Approved, Rejected, Draft, Confidential, For Review. Custom stamps can be created from any image or text.

Tool 1: Adobe Acrobat Reader (Free)

Adding a highlight:

1. Open PDF in Acrobat Reader
2. Select text with mouse
3. Right-click → Highlight Text

Adding a sticky note:

1. Open the Comment tool from the toolbar
2. Click where you want the note
3. Type your comment

Viewing all comments: Open the Comments panel on the right — lists all annotations chronologically, searchable by keyword.

Limitation: Free Acrobat Reader cannot save annotations to PDFs that were not rights-enabled by the creator. If you encounter this, use Foxit Reader instead.

Tool 2: Adobe Acrobat Pro

Adds beyond the free Reader:

• Drawing tools (lines, arrows, shapes)
• Custom stamps from any image or text
• Comment summarisation — export all comments as a separate PDF
• Review workflows — send for shared review, collect feedback from multiple reviewers
• Measurement tools for technical drawings

Tool 3: Foxit PDF Reader (Free)

Strong annotation support at no cost:

• Highlight, underline, strikethrough
• Sticky notes and text boxes
• Drawing tools (lines, shapes, freehand)
• Custom stamps
• Typewriter tool — add text directly on the page

Available at foxit.com. Generally faster than Acrobat on large documents.

Tool 4: Web Browsers (Chrome/Edge)

Both have built-in PDF viewers with basic annotation:

1. Open PDF in Chrome or Edge
2. Click the Draw or Highlight icon in the PDF toolbar
3. Draw directly on the document
4. Ctrl+S to save with annotations

Limited to freehand drawing — useful for quick markups without installing anything.

Tool 5: Preview on Mac

1. Open PDF in Preview
2. Click Show Markup Toolbar (pencil icon)
3. Available: Highlight, Freehand draw, Shapes, Text, Signature, Speech bubble

Annotations are stored in the PDF itself, compatible with other viewers.

Annotation on Mobile

iPhone/iPad:

• Adobe Acrobat for iOS: highlight, comment, draw — syncs with desktop
• Files app: tap the markup icon for drawing and highlighting

Android:

• Xodo PDF Reader: full annotation suite with real-time collaboration via shared links
• Adobe Acrobat for Android: same as iOS version

Saving and Sharing Annotated PDFs

Most viewers save annotations within the PDF file. The annotated PDF can be shared like any other PDF — annotations are visible in the recipient's viewer.

To make annotations permanent (flatten them): Use Flatten PDF to embed annotations into page content so they cannot be deleted or hidden by recipients.

To export comments only (Acrobat Pro): Comment → Export All To Data File (.fdf) — recipients import this into their copy of the document.

Annotation Best Practices

Use colours consistently: Yellow = key points, orange = questions, pink = action items. Brief your team.

Be specific in comments: "The liability clause in §3.2 conflicts with §7.1 — which takes precedence?" is useful. "See p.5" is not.

Sticky notes vs text boxes: Sticky notes collapse to an icon; text boxes are always visible. Use text boxes only for content that must be seen without clicking.

Flatten before publishing: Remove internal comments before sharing externally, or flatten if you want them preserved but not editable.

Frequently Asked Questions

Will annotations print? By default, yes. To print without: in print dialog, choose Document instead of Document and Markups.

Can multiple people annotate the same PDF? Yes — each annotator adds their own layer. In Acrobat, comments are attributed to the logged-in user. Xodo supports real-time simultaneous annotation.

Are annotations secure? Anyone with the file can view and delete annotations. Delete internal notes before sharing externally.

Summary

Acrobat Reader (free) covers the most common annotation needs: highlights, sticky notes, and text boxes. For drawing tools and custom stamps, use Foxit Reader (free) or Acrobat Pro. Use consistent colour-coding for team reviews, and flatten annotations before sharing publicly. Mobile annotation via Acrobat or Xodo works well on both iOS and Android.