How to highlight text in a PDF

▶ 1. Highlighting or editing the text?

These are two different operations, and picking one at random often means doing the work twice.

• ✓ Highlighting adds a coloured layer above the text: the original stays intact and the highlight can be removed at any time.
• ✓ Changing the text colour modifies the document itself: useful for design work, less so for studying or reviewing.

For studying, re-reading or flagging passages to a colleague, highlighting is almost always the right choice. If instead you are laying out a document and want to change the colour of the words, the other route applies: you will find it in the guide on text colour and background.

▧ 2. Using the highlighter

This is the most direct tool: it behaves exactly like a physical highlighter pen.

1. Install the extension from the download page and open the PDF with Choose a PDF.
2. Switch on Highlighter from the toolbar.
3. Run over the text you want to emphasise: a word, a line or a whole paragraph.
4. Pick the colour you prefer to tell different kinds of remark apart.
5. When you are done, click Download PDF to save the highlighted document.

✎ 3. Highlights as annotations

If the document is going to be reviewed by several people, the annotations panel is the better route: the marks stay editable in compatible PDF readers too.

1. Open Annotations and comments and choose the Annotation type: highlighter, rectangle, circle, check or cross.
2. Draw the mark on the page.
3. Fill in author and subject if others will read the document: that way it is clear who flagged what.
4. Adjust colour and Opacity % to calibrate how intrusive the mark should be.
5. When a remark has been dealt with, mark it as resolved.

🎨 4. Colours and readability

A well-highlighted document reads better than the original; a badly highlighted one becomes unreadable.

• ✓ Give each colour a meaning — say yellow for key concepts, green for definitions, red for doubts — and keep it consistent throughout the document.
• ✓ Avoid colours that are too dark: black text under an intense highlight becomes hard to read.
• ✓ If a shade covers too much, lower the Opacity % instead of changing colour.
• ✓ Highlight sparingly: if half the page is coloured, the highlighting no longer distinguishes anything.

🖨 5. Highlights and printing

A detail that often slips by: annotations can be visible on screen but excluded from printing, or the other way round.

1. In the annotations panel check the Print annotation option.
2. Leave it on if the highlights have to appear on the paper copy as well.
3. Switch it off if they are only for you while reading on screen and the printout has to stay clean.
4. If you print in black and white, remember that different colours can turn into very similar greys: in that case a written note alongside helps.

◎ 6. Scanned documents

On a scan a highlighter that follows the words has nothing to grip, because the text does not exist: there is only the image of the page.

1. Run Local OCR to turn the scan into selectable text: it works offline in 10 languages.
2. After recognition the highlighter behaves as it would on any other PDF.
3. Alternatively, without OCR, you can still mark areas with a rectangle from the annotations panel, adjusting its opacity.

? 7. Frequently asked questions

The questions that come up most often on this topic.