How to Add Comments and Markup to a PDF for Free

You do not need a subscription to comment on a PDF. Annotations are part of the PDF standard, not a paid feature, and several free tools write them correctly.

The part worth understanding first is why comments sometimes arrive on the other side and sometimes do not, because that is the actual problem behind most searches for this.

Two different things people call "marking up"

Annotations live in a separate layer above the page. The original content is untouched. They can be moved, edited, deleted, replied to and listed, and any reader that understands the standard can show them.

Flattening burns your marks into the page itself. They become part of the drawing, indistinguishable from the original content. Nobody can move or reply to them, and nobody can remove them either.

Both have a place. Annotate while a document is under review; flatten only when you issue something and want the marks to be permanent and non-negotiable. Trouble starts when a tool flattens without telling you, which is common in web-based editors and phone apps.

Free ways to do it

• TorReader PDF — highlight, draw, add shapes and notes, on Windows and Linux, no installation. Every markup can carry a text note, and all of them appear together in a Comments list so a reviewer can work down the page rather than hunting for small marks on an A0 sheet.
• Adobe Acrobat Reader — the free Reader does include commenting. Large install with background services, but if it is already on the machine it works.
• Okular — the strongest free option on Linux, with a proper review sidebar. Installs on GNOME too, at the cost of pulling in KDE libraries.
• Preview on macOS — fine for a highlight and a note, limited beyond that.
• Your browser — Chrome and Firefox can view and, in recent versions, add basic markup. Convenient, but the least predictable about what it writes into the file.

What we would avoid: free web tools that ask you to upload the document. Putting a contract or a client drawing on somebody else's server to draw an arrow on it is a poor trade, and many of them flatten and re-compress the file on the way back.

Why comments disappear between apps

Four reasons, in the order they actually happen.

Someone flattened the file. Printing to PDF, exporting, or "saving a copy" in some tools discards the annotation layer or fuses it into the page. The marks may still be visible, but they are no longer comments and cannot be replied to.

The other side is using a viewer that ignores annotations. Some lightweight and mobile viewers render the page and nothing else. The comments are in the file; the viewer simply does not draw them.

The tool wrote a non-standard annotation. A few applications store marks in a private form that only they understand. If your comments look right for you and blank for everyone else, this is usually why.

Two people edited different copies. The commonest of all, and not a technical fault. Two reviewers each downloaded the file, each marked it up, and now there are two versions and no merge. Agree who holds the file before a review starts, not after.

Making a review that works

• One file, one owner. Decide who holds the marked-up copy and route everything through them. Half of all lost comments are a versioning problem, not a software problem.
• Attach a note to every mark. A cloud with no text means "something is wrong here" and nothing else. The person fixing it has to guess, and will guess wrong at least once.
• Work from the comment list, not the page. On a large sheet, small marks are easy to miss visually. A list is the only way to be sure you addressed all of them.
• Do not sign until the review is finished. A digital signature is invalidated by later changes, which is exactly what it is for — how signing works.
• Flatten at issue, not before. Once flattened, nobody can undo your marks, including you.

Annotating large drawings specifically

Marking up a 200-page report and marking up an A0 general arrangement are different jobs. On a drawing you are zoomed deep into a detail, then out to place the next mark, over and over — so what decides whether the work is pleasant is how the viewer handles zoom and pan, not how many markup tools it has.

That is the case TorReader PDF's GPU vector rendering was built for: lines stay crisp while zooming and panning instead of going soft until a redraw finishes. More on that in opening large CAD drawing PDFs.

The honest limits

No free tool gives you a shared review server where several people comment on the same document at once and see each other's marks live. That is what the paid collaboration platforms sell, and if simultaneous multi-party review is your requirement, none of the options here replaces it.

For the far more common case — one document, a handful of reviewers, comments going back and forth — free tools do the whole job, and have for years.