Underline PDF

Runs in browser PDF Editing & Annotation

Select the words and the underline is drawn under every line of the selection in the colour you choose. Saved as a standard annotation readers understand, or flattened into the page.

Privacy: Processed locally in your browser. Your file is not uploaded.

What is a PDF annotation, and why does whiting-out text not remove it?

An annotation is an object attached to a page on top of its content: highlights, underlines, strike-throughs, sticky notes, free text, ink drawings, shapes, stamps and signature fields are all annotation types defined in ISO 32000, stored in the page's /Annots array and editable or removable later. Flattening draws them permanently into the page content. The PDF editor on this site keeps only highlights, underlines, strike-throughs (unless you choose to flatten them) and sticky notes as annotations; the text, drawings, shapes, stamps and signature images it adds are written straight into the page content and are permanent. A white rectangle drawn over text is exactly that — a shape on top — so the text beneath still exists, copies and searches; only redaction, which deletes the underlying content, removes it.

Security note: Covering text with a shape is not redaction: the hidden text remains in the file and can be selected, extracted or found by search. Use a redaction tool that removes content, then check the result by searching for the removed text.

Read the guide: How to Edit a PDF Without Uploading It

How to use Underline PDF

  1. Drop a PDF.
  2. Drag across the words to underline.
  3. Pick a colour.
  4. Click "Save PDF" and download.

Underline PDF features

  • Follows the baseline of every selected line
  • Colour and opacity
  • Standard Underline annotation or flattened
  • Switch to highlight or strikethrough without leaving the page
  • Local processing

Underline PDF example

Underline defined terms

Input:

Drag over each term; dark red

Output:

terms-underlined.pdf — Underline annotations, one per selection

Frequently asked questions about Underline PDF

How is this different from drawing a line?

An underline annotation is attached to the text: it follows the exact baseline of each line, spans several lines when you select a paragraph, and readers list it as a text markup. A drawn line is just a line.

Can I change the colour after drawing?

Yes — select the underline and pick another colour in the toolbar. Opacity is available too.

Is my PDF uploaded to a server?

No. The file is opened and processed by your browser with WebAssembly and JavaScript; nothing is sent to Mutqan Soft or anyone else. Close the tab and the file is gone. Downloading the result needs a free account — no subscription, no payment.

Can I use it on a password-protected PDF?

Yes, if you know the password: the file list asks for it once and every operation then works on the decrypted pages. The output is saved without a password unless you protect it again.

Technical notes

Quads come from the text layer like the highlighter; the appearance stream strokes a line at 7 % of the line height above the bottom of each quad.

References