Highlight PDF
Highlight, underline,
and annotate PDFs
Draw a highlight over any text to mark it up. Switch to underline or strikethrough in one click. Choose any colour and opacity level.
File never leaves your device
Four annotation types
Highlight
Fill behind text with a semi-transparent colour. Adjust opacity from 10% to 100%. The most-used annotation type in tools like <a href="https://www.zotero.org/" target="_blank" rel="noopener">Zotero</a> and <a href="https://www.ilovepdf.com/" target="_blank" rel="noopener">ilovepdf</a>.
Underline
Draw a line under text — ideal for marking important passages without covering them. Supported in all major PDF viewers and <a href="https://helpx.adobe.com/acrobat/using/commenting-pdfs.html" target="_blank" rel="noopener">Adobe Acrobat</a>.
Strikethrough
Cross out text to mark it as deleted or superseded. Common in edits and legal documents.
Freehand
Draw freely over the page — circle items, add arrows, or mark anything that doesn't fit a rectangle. As covered by <a href="https://www.lifewire.com/annotate-pdf-5190498" target="_blank" rel="noopener">Lifewire</a>, freehand is the most flexible annotation option.
Frequently asked questions
Does highlighting change the original PDF text?
No. Highlights, underlines, and strikethroughs are overlaid on the page as annotation objects. The original text content of the PDF is not modified — the annotation references the page geometry but does not alter the content stream.
Can I choose the highlight colour?
Yes — you can pick any colour using the colour picker in the Properties panel, and adjust the opacity from fully transparent to fully opaque. Common academic colours are yellow (primary), blue (secondary), and pink (tertiary), as recommended in Zotero's PDF reader documentation.
Will the highlights appear in Adobe Reader?
Yes. Annotations are embedded as standard annotation objects defined in ISO 32000 (PDF specification), visible in Adobe Reader, Chrome, Firefox, and any standard viewer. Lifewire's guide to highlighting PDFs confirms that web-based highlighters produce compatible annotations.
Is there a free tool to highlight PDFs without Adobe?
Yes — PDFTheGuide is completely free and requires no Adobe software. Other free alternatives include ilovepdf.com (uploads file to server) and PDFGear (desktop app). PDFTheGuide is the only option that keeps your file entirely in your browser.
Can I remove a highlight after placing it?
Yes. Select the highlight with the Select tool and press Delete, or use Cmd+Z (Mac) / Ctrl+Z (Windows) to undo immediately after placing it.
Can I highlight scanned PDFs?
Yes — you can draw highlights over any PDF page regardless of whether it contains selectable text or is a scanned image. The highlight is a geometric overlay, not a text-selection operation.