Highlighting sounds like the simplest thing you can do to a PDF, and it is the operation people most often find mysteriously unavailable. The reason is that there are two completely different things called "highlighting", and which one you can use depends on the document in front of you.
Text highlights versus drawn rectangles
A text highlight annotation is the real thing. It is anchored to specific characters in the document. The viewer knows which words you marked, so the highlight follows the text when the page reflows on a phone, the marked text can be extracted into a list of your highlights, and a search can find it. This requires the PDF to contain actual text objects.
A drawn rectangle is a coloured shape placed over an area of the page. It looks similar and knows nothing. It does not know what is underneath it, will not follow reflowing text, and cannot be exported as a list of quotations. But it works on absolutely anything, including a scan.
If you can select text in the document with your cursor, you can use real highlights. If you cannot, the page is an image and a rectangle is your only option.
Highlighting with what you already have
For a document you are reading rather than producing, the tool you already have is usually the right one.
macOS Preview. Select text and press Cmd-Ctrl-H, or use the highlight button in the markup toolbar. Change colour from the dropdown beside it. Preview also gives you a Highlights sidebar listing every passage you have marked, which is genuinely useful for study.
Chrome and Edge. Both include markup tools in their built-in PDF viewers, including highlighting, and can save the annotated file.
Firefox. The built-in viewer added a highlight tool, with colour choice and a free-form option for scanned pages.
iOS and iPadOS. Open in Files or Books, tap Markup, and use the highlighter. With an Apple Pencil this is the most pleasant highlighting experience on any platform.
Adobe Acrobat Reader. Free, cross-platform, with full annotation support including highlights, sticky notes, and a comment list that summarises everything you marked.
For reading and studying, these beat any web tool: no upload, no limits, and your annotations save straight back into the file.
Highlighting in the browser with an editor
When you are marking up a document to send back — flagging clauses in a contract, calling out sections for a colleague — a browser editor like Docento.app handles it as an overlay:
- Open the PDF in the editor.
- Place a coloured box over the region you want to mark.
- Reduce its opacity so the text underneath stays fully readable. This is the step that separates a useful highlight from an unreadable block of colour.
- Add a text note beside it if the highlight needs explaining — a mark with no comment often leaves the recipient guessing what you meant.
- Export.
Because the marks become part of the exported file, recipients see them in any viewer with no special software. Everything is processed on your own machine; see privacy in browser PDF editing.
The trade-off is the one described above: this is a rectangle, not a text-anchored annotation. For marking up a document you are sending onward, that is all anyone needs. For building a study system where you extract your highlights later, use a reader that supports true text highlights.
Using colour deliberately
A colour system turns highlighting from decoration into a filing mechanism. A common convention:
- Yellow — key points and general interest
- Blue — definitions and terminology
- Green — agreed, approved, or action items
- Pink or red — errors, concerns, and things needing follow-up
The specific scheme matters far less than using it consistently. What kills the value of colour-coding is inventing a new meaning each session, so that six months later a green mark tells you nothing.
Two practical cautions. Around 1 in 12 men has some form of colour vision deficiency, and red-green distinctions are exactly the pair most commonly affected — so if the document is going to a colleague, do not let colour carry meaning on its own. Add a short text note. And if you are sending marked-up documents to others regularly, include your key on the first page; nobody can guess your system.
Highlighting well
Under-highlight. A page with half its text marked conveys nothing. The value of a highlight is entirely in its scarcity. If you find yourself marking most of a paragraph, mark the one sentence that carries it instead.
Mark the claim, not the paragraph containing it. Precision at the sentence level makes a re-read far faster than a block covering half a page.
Write down why. The highlight records that something mattered; a two-word note records why, which is the part you will have forgotten. This is the single highest-value habit in document markup.
Clean up before sharing. Your private reading annotations — the confused question marks, the "check this" notes — are not what you want a client seeing. Review before you send.
Do not use a highlight to hide anything. A coloured box over sensitive text obscures it visually while leaving the characters in the file, fully copyable. That is not redaction; it is a widely-repeated mistake that has leaked real documents.
Sharing marked-up documents
Once exported, annotations are baked into the file and every recipient sees them. If you have highlighted a large scanned document, the result may be big enough to bounce off a mail server — compressing it usually fixes that.
For the wider set of markup tools and how to use them in review workflows, see the annotating a PDF guide and the beginners guide to editing PDFs.
