Add underlines to text in PDF documents with custom colors.
Drop PDF here or click to open
Supports PDF files up to 100MB
Select the PDF to underline.
Select text and apply underline.
Save your annotated document.
Underline words in a PDF when you need to mark a clause, citation, definition, or sentence without covering the original text. Drag across the exact text, choose a color, and review the annotation before downloading.
The underline is saved as a PDF text-markup annotation rather than a painted line. Use strikethrough for proposed deletions and highlight for passages that need broader visual emphasis.
Yes. OnePDFs is free to use, requires no account, and does not add an extra OnePDFs branding watermark; watermark tools add only the watermark you choose.
Your files stay on your device and are processed locally in your browser; no upload to our PDF processing service is required. A browser copy can remain in this site's IndexedDB until you clear site data or the browser evicts it.
The tool supports PDFs up to 100MB. Work runs in your browser, so the time depends on page count, document complexity, and your device resources.
No installation is needed. The tool runs through your browser and does not send the PDF to our processing service.