Underline Text in PDF

Underline text in PDF documents online. Add underline annotations to mark key passages. Free, no download.

About PDF Underline

Underline text in PDF documents directly in the browser — select any text passage and apply underline annotation with a single click. PDF AI Tools text underline tool lets you mark up PDFs for study, review, or annotation without software installation.

Underline is a distinct annotation type from highlight — useful for marking references, titles, and key terms with a visual weight that differs from colored highlighting.

How to Use Underline Text in PDF

  1. Step 1: Upload your PDF to the Edit PDF tool
  2. Step 2: Select the Underline annotation tool
  3. Step 3: Click and drag to select text for underlining
  4. Step 4: Download the annotated PDF

Key Features

How We Compare

Compared to desktop alternatives like Adobe Acrobat Pro (starting at $19.99/month), Smallpdf ($12/month for unlimited), or iLovePDF ($9/month Premium), PDF AI Tools delivers comparable quality at $0 for the core feature set. We skip the subscription friction by processing most operations directly in your browser with WebAssembly — no server infrastructure costs to pass on to users. Our AI features (summarization, chat, OCR) use a pay-as-you-go backend that keeps your total cost well under $5/month even for power users.

Frequently Asked Questions

What is the difference between highlight and underline?

Highlight applies a colored background fill. Underline draws a line beneath the text. Both are standard PDF annotation types.

Can others see my underlines?

Yes — underlines are embedded as standard annotations visible in all PDF readers.

Can I remove an underline after saving?

Underlines can be removed by reopening the PDF in the edit tool and deleting the annotation.

Does underline work on scanned PDFs?

For scanned PDFs, use OCR first to make text selectable, then apply underlines.

Who Uses This Tool