Highlight PDF FREE | PDF Highlighter Tool Online

Open any PDF, pick a highlight colour, set the brush height, and drag across any line of text to highlight it in a perfectly straight horizontal band. Navigate all pages, undo any highlight, and download the finished PDF. Free, private, runs in your browser.

Private - No Upload

Straight Lines Only

Any Colour

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PDF Highlighter

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PDF files only

Frequently Asked Questions

Open your PDF using the button above, then pick a colour and drag left or right across any line of text. The highlight snaps to a perfectly straight horizontal band at the height where you clicked. Use the Line Height slider to match the height to the text size in your document. When you are done, click Download PDF to save the highlighted copy.

Real highlighter pens draw straight horizontal bands across text lines. This tool works the same way: when you click and drag, the highlight locks to the exact vertical position where you pressed and only extends horizontally as you move left or right. This means every highlight is perfectly aligned with the text, just like marking a physical document with a pen highlighter.

Yes. You can switch colour between every stroke. Six preset colours are available: yellow, green, pink, blue, orange, and cyan. Click the rainbow circle to open a full colour picker and use any colour you want. Colour coding different types of content is a great way to organise notes and make review faster.

Click Undo to remove the most recent highlight one at a time. Switch to Eraser mode and drag across any highlight to remove it precisely. The Clear button removes all highlights from the current page. Each page has its own set of highlights so clearing one page does not affect any other page.

Yes. Use the left and right arrows to move between pages. Highlights on every page are remembered as you navigate. The downloaded PDF includes all highlights on all pages in one file.

No. Your PDF is opened and processed entirely in your browser using PDF.js, the same library used inside Mozilla Firefox. Nothing is sent to any server at any point. Your document stays completely private on your own device.

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One Doc provides Forever Free browser-based tools. Nothing is uploaded. Your files never leave your device.

Free PDF Highlighter Online - Mark Up Any PDF with Straight Highlight Lines

Highlighting a PDF is one of the most common tasks in study, research, legal work, and professional document review. Students mark key passages in textbooks and lecture notes. Researchers identify critical sections in academic papers. Lawyers annotate contracts and case documents. The PDF Highlighter on OneDocPDF works exactly like a physical highlighter pen: click where you want to start, drag left or right, and a perfectly straight horizontal band of colour covers that line of text. The highlight locks to the vertical position of your click and only moves horizontally, so every stroke is aligned with the text no matter how fast you drag.

The line height slider controls how tall the highlight band is. For standard body text in a typical document, a height of 14 to 20 pixels covers one line comfortably without overlapping the line above or below. For larger heading text, increase the height to 24 to 36 pixels. For dense small-print documents, reduce to 10 to 12 pixels to stay within a single line. The height is shown in pixels relative to the rendered page size on screen, so the same setting will highlight the same proportion of the page regardless of your screen resolution.

Colour coding is one of the most effective study and review strategies. Using different colours for different categories of information makes the document much easier to scan during revision. For example: yellow for key definitions, green for examples, pink for questions to revisit, blue for supporting evidence, and orange for dates or names. This tool lets you switch colour between every single stroke so you can apply any coding system you already use or want to establish.

The downloaded PDF is produced by rendering each page at 2x resolution with all highlights merged into the image, then assembling all pages into one complete PDF using jsPDF. The highlights are permanently embedded and display correctly in any PDF viewer on any device. The output file can be printed, emailed, shared, uploaded to a learning management system, or stored in a document archive.

For other PDF tools see our PDF signature tool, add header and footer to PDF, PDF watermark tool, and PDF compressor. Browse all free PDF tools and image tools on OneDocPDF.