Is a PDF comment part of the page?

No. A comment is an annotation: an object that sits on top of the page and is listed separately in the reader's comments panel. Anyone with the file can hide it, move it, reply to it or delete it, and the page underneath is unchanged.

Why do my comments show my computer's user name?

Most PDF apps fill in each comment's author field automatically from the account you are logged in to, and that name travels with the file. Check the author name in your app's preferences before you mark up anything you will send outside your organisation.

Can a comment reveal text that was redacted?

Yes. A comment is stored separately from the page, so a highlight or note that quotes a passage keeps quoting it even after the passage itself has been blacked out. Deal with comments before redacting, or use a redaction tool that checks them.

Guide

Annotating a PDF Without Uploading It — and What Your Comments Reveal

A draft contract, a thesis chapter, a report due on Friday: the documents people mark up are usually unfinished, often confidential, and rarely meant for anyone but the author and the reviewer. Uploading one to a free annotation website to highlight three sentences hands the whole draft to a server you know nothing about. This guide covers how PDF markup actually works, what the person you send it to will see, and the one detail most people never notice: the name every comment carries.

Comments sit on top of the page

A highlight in a PDF is not paint on the page. It is an annotation: a separate object that says "draw a yellow band over this area, and here is the comment that goes with it". The page underneath is unchanged. That is why a PDF reader can list every comment in a panel, jump to each one, hide them all with one click, or let the recipient reply to a note and mark it resolved.

The standard kinds cover most reviewing: highlight, underline and strike-through, which follow lines of text; notes, the small speech-bubble icons that open to show a comment; and drawn marks such as boxes, arrows and freehand ink. Every one of them can carry a comment, so a highlight can say "reword this" and a struck-out sentence can say why.

Highlighting needs real text

Text markup snaps to words because a text-based PDF knows where each word is. A scanned page does not: it is a photograph of text, and there is nothing to snap to. On a scanned page you can still drag a box over an area, but it will not follow the lines. The scanned vs text-based PDFs guide shows how to tell which kind you have in five seconds.

What the recipient sees

Acrobat, Preview, Edge and Chrome all show standard PDF comments, and all but the simplest viewers list them. Each entry shows the kind of mark, its comment, when it was made — and who made it. Because comments are separate objects, the recipient can also move or delete them, so a comment is a conversation, not a seal: if a mark must stay put, it needs to be drawn into the page instead.

Comments or flattened markup

Flattening draws the marks into the page itself. They then look identical in every viewer and cannot be dragged off or hidden with a click — but they stop being comments. There is no list to work through, nothing to reply to, and a note has nowhere to go, so its text is lost. Flatten when you are sending a finished, marked-up copy for the record; keep comments when you want the author to work through them. Flattened markup is harder to remove, not impossible: anyone with a PDF editor can still cover or crop it.

The name on every comment

Every PDF comment has an author field, and most desktop PDF apps fill it in for you, silently, from the user account on your computer. Send a marked-up draft to a client and every note may say "jsmith-laptop" or your full legal name, whether you meant to share it or not. It is the same kind of leak as the document properties described in the guide to PDF metadata: invisible on the page, obvious to anyone who looks.

Each comment also records the time it was made. Apps usually write your local time with its offset from UTC, which quietly tells the recipient your time zone.

Comments can outlive a redaction

A comment keeps its own copy of what it says. A highlight whose comment quotes a client's name, or a note that repeats an account number, still says so after the passage it points at has been blacked out. If you are going to redact a document, deal with its comments first — the redaction guide explains why covering text is not the same as removing it.

Marking up without uploading

The annotate tool adds highlights, underlines, strike-throughs, pen drawings, boxes, ellipses, arrows, text boxes and notes as ordinary PDF comments, or draws them into the page if you tick Flatten, entirely in your browser. The author name is blank unless you type one, and it is never saved — reload the page and it is gone. Comment times are saved in UTC. Comments already in the file are kept exactly as they were. Nothing is uploaded.

A quick checklist

Check whose name your PDF app puts on comments before you start.
Write comments you would be comfortable with the author forwarding.
Open the saved copy and read the comments panel before sending it.
Remove comments before redacting, or check them afterwards.