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Editing & Annotation

How to Annotate a PDF (Highlight, Comment, Mark Up)

The short answer

Annotating a PDF means adding your marks on top of it — highlights, comments, underlines, sticky notes, drawings. It’s the easiest PDF skill there is: free tools everywhere, no file changes, and you can undo it all. Preview on Mac, your browser, or any reader with a comment tool does it.

Annotate a PDF on a Mac

  1. Open the PDF in Preview.
  2. Click the markup toolbar (pencil in a circle).
  3. Pick your weapon: highlighter, underline, text box, note, shape, or signature.
  4. Mark up the page, then File → Save.

That’s it. Highlighting text, adding a sticky note, circling a figure — all built in, all free, all reversible.

Annotate a PDF in your browser

Chrome, Edge, and Firefox all have built-in PDF viewers with annotation tools now: open the PDF, click the markup icon, and highlight or draw. For richer annotation — comments that multiple people can see and reply to — use a document tool: Google Drive’s viewer, or a collaboration tool like Kami or Hypothesis. That’s the difference between marking up for yourself and reviewing as a team.

Annotate on your phone

iPhone: the Files app opens PDFs with a markup toolbar (pencil icon) — highlight, draw, sign, all with your finger. Android: Google Drive’s editor does the same. Phones are genuinely fine for light annotation; heavy review on a small screen is misery.

The honest part

Three things worth knowing:

  • Annotations are layers, not edits. You’re marking on top of the page — the original content stays untouched. That’s why it’s safe and reversible. If someone later flattens the PDF, the marks become permanent, so flattening a document with your notes is a one-way door.
  • Highlighting doesn’t work on scans. A scanned page has no text to highlight — the highlighter tool on a scan acts like a marker pen, painting over pixels. Same reason searchable text needs OCR.
  • Saved or not? If you annotate in a viewer that auto-saves, your marks may already be stored in the file without you clicking Save. Check before you send a “clean” document to someone — your margin comments might travel with it.

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