Underline Text - Free Online Browser Tool

Add markup underline annotations to text sections.

Underlining is the subtlest of the review marks — it flags something without shouting about it, which is why it is used for definitions, citations and clauses that need a second reading. This tool applies proper underline markup to the pages you choose.

This underline text utility runs completely inside your web browser. Your document is opened locally and processed on your own device using JavaScript and WebAssembly, so your file is never uploaded to any server. There is no signup, no watermark and no limit on how many documents you can handle.

  • 100% client-side - files never leave your device
  • Instant processing, no upload wait
  • Works on desktop, tablet and mobile
  • Free forever, no account required

Common next steps: Highlight PDF, Add Sticky Notes / Comments, Recolour Annotations.

How to Use the Underline Text Tool

Four steps, and the whole thing runs locally on your own device.

  1. Open your document

    Drop in the PDF to review.

  2. Choose the pages to mark

    Select a range, a single page or the whole document depending on where the underlined content sits.

  3. Apply underline markup

    Underline annotations are added in the standard interoperable format, so other readers display them correctly.

  4. Download the marked-up file

    Save the document with its markup intact for the next reviewer.

Frequently Asked Questions

How is underline markup different from drawing a line?

Underline is a text markup annotation tied to the text layer, so it follows the text and is understood by other reviewers’ tools. A drawn line is just ink on the page.

Will the underline be visible if I flatten the document?

Yes. Flattening bakes annotations into the page content, so the underline remains, but it stops being an interactive annotation.

Can I underline only some words on a page?

This tool applies markup at page level. For word-level precision, use the underline tool in your PDF reader’s annotation mode instead.

Does the underline affect the file size?

Negligibly — markup is lightweight and does not re-encode page content.

What is the usual convention for underlines in review?

Underline commonly flags text that needs attention, drafting language or a defined term, as distinct from a highlight which flags something to discuss.