How to underline text in a PDF

The fastest way: draw a line under the text

PDFs do not store underline formatting the way a word processor does, so the reliable way to underline is to draw a thin line beneath the words. The line tool gives you a crisp, straight underline that looks intentional.

Because everything happens locally in your browser, your document is never sent to a server — useful when the file is a signed contract or private study material.

• Open your PDF in the editor (drag and drop or pick the file).
• Scroll to the text you want to emphasize.
• Select the line tool, then click at the start of the word and drag to the end, just below the baseline.
• Release the mouse to set the underline. Repeat for any other passages.
• Click Download to save the edited PDF to your device.

Use freehand draw for a quick, hand-marked look

If you want the look of a pen mark — the kind you would make on a printed page while revising — the freehand draw tool is faster than aligning a perfectly straight line. Hold the mouse and trace under the words.

This is handy for study material and proofreading, where a slightly imperfect underline reads as a personal annotation rather than a formal edit. You can adjust the color and thickness before you draw.

Tips for a clean, straight underline

• Place the line just below the text, not through it, so letters with descenders (like g, y, p) still read clearly.
• Keep the thickness modest — a thin line looks like an underline; a thick one starts to look like a strike or a highlight bar.
• Zoom in before drawing for more precise placement, especially on small body text.
• For multiple lines of text, draw one underline per line rather than one long diagonal across the paragraph.
• Use undo if a line lands crooked, then redraw it — no need to start over.

Highlight instead, when underlining is not enough

Sometimes a colored band carries more emphasis than a thin line — for example, flagging a clause you want a counterparty to notice, or marking key definitions in lecture notes. The highlight tool draws a translucent color over the text so the words stay readable underneath.

Underline and highlight can be combined on the same document: underline the exact phrase that matters and highlight the surrounding sentence for context. Both are non-destructive layers added on top of the page, and both download into the final PDF.