Underline words in a PDF in 4 steps
- 1
Open Underline PDF
Go to Underline PDF and select the file. The Underline tool is active.
- 2
Choose a color
Underlines start red. Set Color and Opacity in the side panel before dragging; navy or black looks more formal on printed documents.
- 3
Drag across the words
Drag from the first word to the last. A preview shows the line as you move. Across several lines, each line gets its own underline.
- 4
Save PDF
Click Save PDF. The file downloads ending in -underlined.pdf.
Line position and thickness
The tool reads where each line of text sits and places the underline slightly below the baseline, so it clears the letters without touching descenders on most fonts. Thickness is set automatically at about 7% of the font size: roughly 0.8 pt under 11 pt body text and thicker under a heading.
To change it, switch to Select, click the underline and use the Thickness slider (0.5 to 6 pt). The slider only appears once an underline is selected, because new underlines always start from the font size.
Underlining on scanned pages
When a page has no selectable text, nothing can snap. Your drag becomes a box, and the underline is drawn along the bottom edge of that box at 1.5 pt. Drag from just above the words to just below their baseline so the line lands in the right place. For a perfectly straight rule that you can angle or extend, Add Line gives you full control over both ends.
When to underline instead of highlight
- Printing in black and white: a dark underline stays clear, while a yellow highlight fades to light gray.
- Dense legal or academic text: an underline marks a phrase without coloring the whole line, which keeps the page calm.
- Proposed insertions: some reviewers underline new wording and strike through the wording it replaces.
- Long passages you will reread: highlighting is easier to spot when scanning pages quickly.
How the underline is saved
The underline is drawn into the page as a vector line. Every viewer shows it and it prints, but it is not an underline annotation, so the text is not linked to it and it does not show up in a comment list. Add a comment next to it if you need to explain why the text is marked.
Frequently asked questions
Can I underline with a double line?
Not in one step. Drag a second underline, select it and nudge it down with the arrow keys.
Why is my underline in the wrong place?
The page probably has no selectable text, so the line follows the bottom of your dragged box. Undo and drag a box that ends at the text baseline.
Does underlining change the text?
No. The words stay the same and remain selectable. Only a line is added under them.