Annotate a PDF online
Highlight passages, draw freehand, box a figure and add margin notes on any PDF. Marks are drawn into the page, so every reader shows them.
- Runs in your browser
- Files are never uploaded
- Free, no account
Mark up a document the way you would with a pen: highlight passages, draw freehand, box a figure, add a margin note. Everything happens in this browser tab — you can turn off your network connection first and it will still work.
Marking up a document without a review server
Commenting on a PDF is normally either heavyweight — a review platform, accounts for everyone, a shared workspace — or unreliable, because a comment left in one reader may not appear in another and frequently does not print at all. Both problems come from the same place: PDF comments are annotations, objects that live alongside the page rather than in it, and every reader treats them differently.
This tool sidesteps the whole category. Your highlighter strokes, pen marks, boxes and notes are drawn into the page content itself. There is no comment pane to enable, no author identity attached, nothing that can be toggled off, and nothing that fails to print. You get a marked-up PDF that behaves like a marked-up piece of paper.
A workflow for reviewing a draft
- Read first, mark second. Go through once with the highlighter picking out the passages you will comment on, then go back and write.
- Use colour as a code. Yellow for "look at this", red for "this is wrong", green for "agreed, no action". Say what the code means in a note on the first page — nothing in the file records it for you.
- Put the note in the margin, not over the text. Click in the white space and type; drag a line from the note to the passage if the connection is not obvious.
- Box figures and tables rather than highlighting them. A highlight over a chart darkens it; an outline box beside it reads cleanly in print.
- Number your points. "1", "2", "3" in the margin, with the detail in a covering email, keeps a dense page legible.
Drawn marks versus PDF annotations
| Drawn into the page (this tool) | PDF annotation objects | |
|---|---|---|
| Shows in every reader | Always | Usually, but appearance varies |
| Prints | Always | Depends on the reader's print settings |
| Survives conversion to image or upload to a portal | Yes | Often lost |
| Recipient can delete individual marks | No | Yes, one click each |
| Threaded replies, author names, timestamps | No | Yes |
| Requires an account or a server | No | Only for shared review |
If you have been sent a PDF that already carries annotations from someone else's reader and you want them to stop being fragile, Flatten PDF draws each one into the page using its stored appearance.
Drawing well with a stylus
The freehand pen listens to pointer events, so an Apple Pencil, an S Pen, a Wacom tablet or a Surface Pen all work. Two things make the result look deliberate rather than shaky: zoom in before you draw, and set the stroke a little thicker than feels right — 2 to 3 points reads as a confident pen mark, while 1 point looks like a scratch once the page is scaled down for printing.
Strokes are simplified on release: points closer together than 0.8 points are dropped. That keeps a page of dense annotation from bloating the file without visibly changing the line.
Privacy when the document is confidential
The documents that most need annotating — a contract in negotiation, a manuscript under embargo, a patient record, an internal audit — are exactly the ones that should never be uploaded to a free web tool. Nothing here is transmitted: the file is parsed in your tab, marked up in your tab, and written back out in your tab. No author name, no device identifier and no timestamp is added to the file unless you type one yourself.
Questions
Are these real PDF comments or drawn marks?
Drawn marks. A real PDF comment is an annotation object that lives beside the page, and readers disagree wildly about how — or whether — to display and print them. Everything you add here becomes page content, so it survives printing, emailing, uploading to a court filing system and opening in a phone's built-in viewer.
Can the person I send it to remove my markup?
Not with an ordinary reader. Once the marks are part of the page content there is no annotation to delete, no comment pane to clear and no layer to toggle off. Someone with a full editor could still paint over them, so this is durability, not security.
Can I reply to comments other people left?
No. Threaded review workflows need a shared server to hold the conversation, and we do not have one — that is the trade for never receiving your document. If you have been sent a PDF with existing comments, Flatten PDF will bake them into the page so they cannot be lost.
Does marking up a legal document leave a trace?
Only the marks you made. There is no author name, no timestamp and no revision history written into the file, because none of that is collected. Your original PDF is untouched on disk; the annotated copy is a new file created when you press download.
Can I annotate on a tablet with a stylus?
Yes. The freehand pen uses pointer events, so an Apple Pencil, an S Pen or a Surface Pen works as smoothly as a mouse — and considerably more accurately. Pinch-zoom the page first if you need fine control.