How to annotate a PDF (highlight and comment, like Adobe)

Last updated: July 7, 2026

Everyone has improvised PDF feedback at some point. You print the contract, scrawl on it, photograph the pages and email them back. Or you write "page 4, third paragraph, the notice-period clause is wrong" and hope the other person finds the right sentence. It works, barely, until a second round of changes arrives and nobody remembers which comment referred to which version.

The proper fix has existed for decades: PDF annotations. A highlight that sits on the exact words, a note pinned to the exact spot, each stamped with a name and a date. Adobe Acrobat does this well, but a licence is overkill for marking up the occasional document. A modern browser can do the same job, with the same standard annotations, for free.

Annotate a PDF in your browser, step by step

robinpdf's Annotate PDF tool is a full PDF viewer with three modes: Read, Highlight and Comment. Here is the whole workflow:

1. Open the Annotate PDF page and drop your file in. It opens instantly, because nothing is uploaded anywhere.
2. Read normally first. You can page through in single or two-page view, which is handy for contracts laid out as spreads.
3. Switch to Highlight mode and select the text you want to mark. There are five colours, so you can separate different kinds of remark (more on that below).
4. Switch to Comment mode and click anywhere on the page to drop a note pin, exactly like Acrobat's sticky notes. Type your remark into the note.
5. The first time you comment, the tool asks for your name and remembers it locally. Every note is stamped with your name and the date, so whoever opens the file later knows who said what, and when.
6. When you are done, export: a normal PDF with your highlights and comments embedded in it.

That exported file is the important part. The marks are not pictures pasted onto the page; they are real, standard PDF annotations, which changes what you can do with the document afterwards.

Real annotations vs marks flattened into the page

Plenty of free tools let you "draw" on a PDF, then bake those drawings into the page image when you save. The result looks fine but behaves like a scan: the highlight cannot be removed, the note cannot be read as a comment, and Acrobat's review panel shows nothing at all. Your feedback arrives as decoration rather than data.

robinpdf writes genuine annotation objects instead: a standard Text annotation for each sticky note, and a standard Highlight annotation with QuadPoints (the coordinates that tie the mark to the exact words) for each highlight. In practice that means:

• They open everywhere. Adobe Acrobat, macOS Preview and most other readers list your comments in their annotation sidebar, attributed to you by name.
• They stay editable. Anyone can delete a highlight, reword a note or add their own, in any compliant reader, without touching the underlying document.
• The document stays intact. The text underneath remains selectable and searchable; you have added a layer on top, not repainted the page.

One honest caveat: annotations are a markup layer, not an editing tool. Changing the actual words of the document is a different job, and permanently removing sensitive text calls for redaction instead. A highlight or a note never hides anything.

Round-trip review: one file, passed back and forth

Because the annotations are standard, the file itself becomes the collaboration. Say you are reviewing a supplier contract with a colleague. You highlight the three clauses that worry you, pin a comment on each explaining why, and email the PDF back. Your colleague drops the same file into the viewer, sees every note exactly where you left it, replies with notes of their own (stamped with their name), deletes the one that is resolved, and sends it back. No shared drive, no accounts, no review platform to sign up for: the PDF carries the whole conversation.

The same loop works for marking up a student essay: highlight the passage, pin the correction next to it, return one file instead of a page of numbered remarks. Or for design feedback, where a pin dropped precisely on the offending logo says more than a paragraph of description. If the feedback spans several documents, merge them into one PDF first and annotate the lot in a single pass.

A colour system that actually works

Five highlight colours are more useful than they sound, but only with a convention. Pick one before you start and state it in your first comment. A scheme that holds up well in contract and copy review:

• Yellow for anything that simply needs attention or a read-through.
• Red for errors and dealbreakers that must change.
• Green for passages you are explicitly approving, so silence is not ambiguous.
• Blue for questions: things you do not understand rather than things you dispute.
• Purple (or whichever is left) for style and wording suggestions the author may freely ignore.

The pay-off comes on the second pass: your colleague can deal with every red mark first and leave the purple ones for the end, without reading a single note twice.

Why doing this in your browser matters

Think about what people actually annotate: draft contracts before signature, unpublished manuscripts, students' graded work, designs under NDA. These are precisely the documents you should not be uploading to an anonymous server just to add a few notes. robinpdf's viewer runs entirely on your device: the PDF is opened, marked up and exported without ever leaving your machine, and even the name stamped on your comments lives locally in your own browser, not in anyone's database. For the longer reasoning, see how to judge whether an online PDF tool is safe.

Tips for cleaner markup

• Read first, mark second. A full pass in Read mode before you touch Highlight saves you from marking things the author fixes two pages later.
• One thought per note. Ten short pinned comments are far easier to resolve one by one than a single essay-length note.
• Highlight the words, comment the reason. A bare highlight forces the reader to guess what bothered you; pair it with a pin whenever the problem is not obvious.
• Set your real name. The name you enter is what Acrobat and Preview will display as the author, so "asus-laptop" is not a great look on a client contract.
• Mind the file size. Annotations add almost nothing, but if the original is a 40 MB scan, compress it before emailing it around.

Frequently asked questions

Will my comments show up in Adobe Acrobat?

Yes. They are exported as standard PDF annotations, so Acrobat, macOS Preview and other readers show them in their comment panels, attributed to the name you set.

Can someone else edit or delete my annotations?

Yes, and that is by design. Annotations are an editable layer, so a reviewer can resolve, reword or remove notes. If you need marks nobody can alter, annotation is the wrong tool.

Can I reopen a PDF that was already annotated?

Yes. Drop it back into the viewer and every existing comment and highlight appears where it was left, ready to edit, delete or add to. That is what makes back-and-forth review work.

Does highlighting work on scanned PDFs?

Text highlighting needs selectable text, so a pure image scan cannot be highlighted word by word. Comment pins still work anywhere on the page, so you can annotate scans with notes.

Do we both need an account to collaborate?

No. The annotations travel inside the PDF itself, so any two people with a browser (or any standard PDF reader) can pass the file back and forth.

Is anything uploaded to a server?

No. The viewer runs entirely in your browser; the document, your marks and your author name all stay on your device.

Annotate your PDF now →