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PDF Annotate — Highlight, Underline, Strike, Note

Add highlights, underlines, strikethroughs, and sticky notes to a PDF in your browser

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About this tool

PDF Annotate gives you a quick palette to mark up a PDF without installing Acrobat or paying for a desktop reader. Drop your file, the tool renders every page with pdf.js, then choose a marker — yellow highlight, underline, strikethrough, or sticky note — pick a colour, and click-and-drag (or click for a note) anywhere on the page. Each mark is staged on a transparent overlay so you can see your work in real time and remove any annotation by clicking it. When you're satisfied, click Save Annotated PDF and pdf-lib bakes every annotation directly into the page using semi-transparent rectangles for highlights, thin lines for underlines and strikes, and a small icon plus colour swatch for sticky notes. The result is a flattened, viewable-anywhere PDF — no proprietary layer, no Adobe-only annotations, just baked-in graphics that any reader can display.

Why use it

  • Four Markers in One Tool: Highlight, underline, strikethrough, and sticky notes covering the four most common review actions in any PDF reader.

  • Colour Picker: Six preset colours plus a custom picker so coloured-coded reviews (yellow = approved, red = problem) work out of the box.

  • Flattened Output: Annotations are baked into the page, so the file looks identical in any PDF reader — no Adobe-only annotation layer.

  • Browser-Only Workflow: pdf.js and pdf-lib both run on-device — drafts, contracts, and reviewed academic papers stay on your laptop.

  • Click-to-Remove: Misplaced an annotation? Click it again to delete before saving — staged edits never commit until you download.

  • No Watermark or Signup: Mark up as many PDFs as you like with no account, no quota, and no UtilityKit branding on output.

How to use

  1. Drop or select the PDF you want to annotate — pages render in a scrollable preview.
  2. Pick a tool from the palette: highlight, underline, strikethrough, or sticky note.
  3. Choose a colour — yellow for highlights is the default; click any swatch to switch.
  4. For highlight / underline / strike: click and drag across a passage of text on any page.
  5. For sticky notes: click anywhere on a page, type your comment, and click Save.
  6. Click any existing annotation to remove it; click Save Annotated PDF to download a flattened result.

When it helps

  • When peer-reviewing a draft research paper, highlighting key arguments and adding margin notes.
  • When approving a contract, marking redline edits and striking through clauses you want removed.
  • When marking up a printable manual or workflow PDF for a colleague who'll print and follow it.
  • When teaching from a PDF textbook chapter and wanting to highlight discussion points before class.
  • When triaging a bug report bundle by colour-coding pages — yellow for to-investigate, red for blockers.
  • When auditing a financial statement and underlining the figures you want to question.

Examples

Reviewing a contract

Input

agreement.pdf — strike the indemnity clause on page 4, add a red sticky note saying 'Discuss with legal'

Expected result

agreement-annotated.pdf with a clean strike-through and a baked-in note marker

Marking up a research paper

Input

paper.pdf — yellow highlight the abstract conclusions, blue underline the methodology heading

Expected result

paper-annotated.pdf ready to email to the co-authors with all marks visible in any reader

Approving a brochure draft

Input

brochure.pdf — green highlights on accepted copy blocks, pink sticky notes on areas needing rewording

Expected result

brochure-annotated.pdf the designer can open in any PDF reader to action the feedback

Tips

  • Use yellow for default highlights and reserve red for the problems you really want to surface — readers scan colour first, text second.
  • For long passages, drag in a single sweep — multiple short highlights look messy stacked.
  • Sticky notes should reference a nearby phrase, not paragraph numbers, so reviewers can find context fast.
  • Strike + sticky note together is the cleanest way to say 'remove this and replace with…'.
  • Save the annotated copy under a new name so the un-annotated original survives for the next reviewer.

Frequently Asked Questions

Are the annotations editable later?
No — they're flattened into the page on save, which is what guarantees they render the same in every reader. If you need re-editable annotations, keep the source PDF and rerun the tool after edits.
Does my PDF leave the browser?
No. pdf.js renders pages and pdf-lib writes the output entirely on your device — no upload, no analytics on file content.
Can I use it on a phone?
Yes, the click-and-drag layer is touch-aware, but precision is easier on desktop. For long passages on mobile, use a stylus where possible.
What colours are available?
Six presets — yellow, green, blue, pink, orange, red — plus a custom picker for any hex colour you need.
Can I make annotations searchable?
Sticky-note text is baked as a small label, so it appears in copy-paste but isn't an Acrobat-style searchable comment. For robust comment workflows, use a dedicated reviewer.
Will the file size grow?
Slightly — each highlight or note adds a small rectangle or text run. For dozens of annotations the increase is usually under 50 KB.
Can I open the result in Adobe Reader / Preview?
Yes — the output is a standard PDF with annotations baked in, so any reader (Adobe, Preview, browsers, mobile) renders them identically.
Is there a sticky-note character limit?
Per note, around 200 characters fit cleanly. Longer notes still save but may overlap nearby content — use multiple notes for very long comments.

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Glossary

Highlight
A semi-transparent coloured rectangle placed over a passage of text to draw attention without obscuring it.
Underline
A thin coloured line drawn beneath text to mark importance without changing readability.
Strikethrough
A horizontal line through text indicating it should be removed or replaced.
Sticky Note
A small marker on the page paired with a comment, used for margin-style feedback.
Flattening
Baking annotations into the page content so they render in any reader, instead of relying on an annotation layer.
Annotation Layer
An optional PDF feature for editable annotations stored separately from the page content; we deliberately flatten instead.