Underlining in a PDF can mean two genuinely different things, and confusing them is the root of most frustration. One is an annotation — a markup line drawn over existing text to flag or emphasize it, the digital equivalent of a highlighter pen. The other is a text-edit, where you change the actual character formatting so the underline becomes part of the document’s content. Which one you need decides which tool works and whether the underline survives editing, copying, or printing.
Annotation Underline vs. Editing the Text
An annotation underline sits on a separate markup layer. It’s ideal for reviewing, proofreading, or marking a contract clause, and it can be removed cleanly without touching the underlying text. It also carries author and timestamp metadata in many readers, which is useful for collaborative review.
Editing the text formatting changes the content itself — the underline behaves like underlined text in a word processor and moves with the words if the layout reflows. This requires a PDF editor with true content-editing capability and only works on PDFs that contain real, selectable text.
Methods for Underlining
Use the underline annotation tool
Nearly every PDF reader — including free ones — has a markup toolbar with an underline option. Select the text, click underline, and a line is drawn beneath the selection. You can usually change its color and remove it later from the annotations list. This is the fastest route and works without editing permissions in many files.
Edit the text directly
In an editor with content editing, click into the text, select the words, and apply underline formatting from the text properties. This is the right method when the underline must be a permanent part of the document — a finalized form label, for example.
Draw a line manually
When the PDF is a scan (an image with no selectable text), you can’t select words to underline. Instead, use the line or drawing tool to place a straight line beneath the relevant area. Hold the shift key in most tools to keep the line perfectly horizontal.
A Practical Workflow
- Determine whether the PDF has selectable text — try to highlight a word. If the cursor selects characters, it’s real text; if it selects a whole block as an image, it’s a scan.
- For review and emphasis, use the annotation underline tool — reversible and non-destructive.
- For permanent formatting, switch to a content editor and apply underline to the selected text.
- For scanned pages, run OCR first if you need selectable text, or draw a manual line if you only need the visual mark.
- Save a copy so the original stays intact, especially when annotations must be preserved separately.
Common Mistakes and Edge Cases
- Trying to select text on a scan: if underline won’t “stick” to words, the page is an image. Run OCR PDF or draw the line manually.
- Annotation vs. content confusion: an annotation underline can be deleted by anyone with the reader’s markup tools; a content-level underline cannot be removed without an editor. Choose based on permanence.
- Misaligned manual lines: freehand lines drift. Use shift-constrain or guides to keep them straight and consistently positioned.
- Flattening removes editability: if you flatten annotations into the page, the underline becomes permanent and can no longer be toggled off.
- Underline vs. strikethrough mix-ups: the markup toolbar groups these together; selecting the wrong one is common when working quickly.
Frequently Asked Questions
Can I underline text in a free PDF reader?
Yes. Underline is a standard annotation feature in most free readers, including the markup tools in common browsers and viewers.
Why can’t I select the text to underline it?
The PDF is likely a scanned image with no text layer. Run OCR to create selectable text, or draw the underline manually.
Will the underline print?
Annotation underlines print in most readers, but some have a “print annotations” setting that must be enabled. Content-level underlines always print because they’re part of the document.
How do I remove an underline later?
Annotation underlines can be deleted from the annotations panel. Content-level underlines require a text editor to remove the formatting.
