How to Highlight Text in a PDF

Highlighting is the most underrated tool in any PDF editor. Drag a yellow streak across a sentence and you've flagged it — for yourself, for a colleague, for whoever opens the file next. No comments, no shapes, no clutter. Just the text that matters made visible. This guide is a focused walkthrough on highlighting text in a PDF using Flint's Annotate PDF tool — when to highlight, how to colour-code without losing your mind, and a few habits worth borrowing from people who read for a living.

When highlighting is the right move

Highlighting wins when you want to mark something without adding commentary. The single act of dragging colour across a phrase says: this matters. Don't scroll past it. Three contexts where it's the obvious choice:

Studying. Textbooks, articles, research papers. You're reading to remember, and highlights are the anchors you'll come back to later. Pair with a summary doc and your retention jumps.
Contract review. Key clauses, payment terms, termination conditions, indemnities. A yellow streak on every term you need to revisit makes the whole document scannable in seconds the second time you open it.
Prepping for someone else. Sending a 60-page spec to a colleague who only needs to read three sections? Highlight those sections first. They'll thank you.

How to highlight text in Flint

Open the PDF in Flint

Head to Annotate PDF and drop the file in. Flint opens it in the editor with the annotation toolbar down the side. Files up to 250 MB on Pro.

Pick the highlight tool

Click the highlighter in the toolbar. Your cursor turns into the highlight cursor — drag it across any text on the page to mark it. The highlight anchors to the underlying text, so it stays put even if the PDF is reflowed or re-rendered.

Choose a colour (and a system)

Default is yellow — the textbook classic. Click the colour swatch in the toolbar to pick another. Stick to two or three colours max per document; more becomes noise. Download when you're done and the highlights survive in every PDF reader.

A colour system worth borrowing

Most people pick a colour at random and end up with a document that looks like a bag of Skittles. A simple, consistent system gives your highlights actual signal. Here are two that work well in practice — adapt one rather than invent your own.

The student's three-colour system

Yellow — important. Things to remember.
Pink or red — critical. Will definitely be on the exam, or the one clause that blocks the deal.
Green — questions. Things you want to ask about or come back to.

The contract reviewer's three-colour system

Yellow — terms to confirm with the other party.
Pink or red — terms you object to or want changed.
Blue — your own internal notes; cross-references and reminders that aren't for negotiation.

Whichever you pick, the rule is the same: agree on the meaning before you start, and don't add a fourth colour mid-document. Discipline beats palette size every time.

Real-world examples of good highlighting

Studying for an exam

Read each chapter through once. Don't highlight on the first pass — you'll over-mark. Read it again and highlight only definitions, key formulas, and turning-point ideas. Use a second colour for things you didn't understand the first time. When you revise, the second-colour highlights become your study list.

Reviewing a contract before signing

Open the PDF. Highlight every clause with a date, a deliverable, a payment term, or a termination trigger. Use a second colour for anything you'll need to negotiate. Hand the marked-up version to your lawyer with a note saying “reds need pushback, yellows are fine.” Saves them an hour of reading.

Pre-reading a long document for a meeting

Skim the document twice. Highlight only the passages you'll reference in the meeting. Print it (or screenshare it) with the highlights visible — your team can follow your reasoning without needing to read the whole thing live.

After you've highlighted

Send it on. Download and email. The highlights are baked into the file as standard PDF annotations — they show up in every reader.
Add the rest of your markup. If highlighting was step one, see the full annotate a PDF guide for rectangles, arrows, sticky notes, and freehand ink.
Sign it if it's for approval. Drop your signature with Sign PDF right next to the relevant highlighted clause.
Lock it. If the highlighted file is sensitive, Password Protect PDF adds encryption before you send.
Combine it with other reviewed docs. Merging marked-up files into a single pack with Merge PDF keeps the handoff tidy.

Other ways to highlight (and where Flint is faster)

Adobe Acrobat Reader

Reader includes a highlight tool. Free to install, slow to start. Fine if Acrobat's already open; not worth installing for the occasional highlight.

Preview on macOS

Built in, free, fast. The highlight tool sits in the markup toolbar. Works well for one-offs. Colour switching is fiddly — you have to right-click each highlight to change it.

Browser's built-in PDF viewer

Chrome and Edge have basic highlight tools now. Convenient, but limited — colours are restrictive and the highlights don't always save back to the file cleanly.

Flint

Browser-based, works on every machine, no install. Highlight tool is one click, colours switch in two. Files stay in your Flint library, so you can pick up the same document on a different machine without re-uploading. Bookmark Annotate PDF if you highlight regularly.

Highlighting habits worth keeping

Don't highlight on the first read. You'll mark everything that sounds interesting and miss what's actually important. Read once, highlight on the second pass.
Less is more. A page with three highlights tells you something. A page with thirty tells you nothing.
Whole phrases, not whole paragraphs. Highlight the part that matters, not the surrounding setup. If you find yourself marking entire paragraphs, you're highlighting in lieu of reading carefully.
Highlight before you compress. Highlights add a small amount of data; compressing afterwards keeps the file size in check. Run Compress PDF as your final step.
Don't use highlights to hide text. Even a fully opaque highlight overlay doesn't remove the underlying text — anyone can still select and copy it. For actual removal use Redact PDF.

Highlighting PDFs: frequently asked questions

Will my highlights survive when I send the PDF?

Yes — they're saved as standard PDF text-markup annotations. Every PDF reader on the planet renders them correctly.

Can the recipient delete or change my highlights?

By default yes, the annotations are editable. If you want them permanent, flatten the file via Edit PDF before sending.

What if the PDF doesn't have selectable text?

Scanned PDFs without OCR don't have selectable text — so highlighting in the conventional sense won't work. You can still draw a coloured rectangle over the region with Annotate PDF, but the result won't flow with the text.

Can I change a highlight's colour after the fact?

Yes. Click any existing highlight in the editor and pick a new colour from the toolbar. Useful when your colour system evolves mid-document and you want to retrofit earlier marks.

Are highlights searchable?

The highlighted text remains selectable and searchable. PDF readers don't typically have a “list all highlights” feature out of the box, but the text itself is fully indexed.

Is it free to highlight?

Trying the tool is free. Downloading the highlighted PDF requires Pro — the same plan that unlocks every download on Flint.

Ready to mark up that doc?

Drop the PDF into Annotate PDF and you'll have the highlight tool in front of you in seconds. Drag, colour, repeat. When you're done, the rest of Flint's toolkit — sign, compress, redact, merge — is one click away.