Annotating a PDF alone is a solved problem and has been for years. Use Preview's Markup on a Mac, the PDF reader in Microsoft Edge on Windows, or the free Adobe Acrobat Reader on either. None costs anything, none needs a login, and all three write the marks into the file. Annotating a PDF with other people is the part that is still broken, because emailing a marked-up file produces two forks and no thread. For that, open the PDF behind a link with an annotation layer over it and send the link, so every comment and its answer stay pinned to the page they are about.
Three readers ship with the machines most people read PDFs on, and all three annotate for free with no sign-in.
macOS: Preview. Open the PDF, click the Markup button in the toolbar, and you get a pen, shapes, a text box, a highlighter, a signature tool and sticky notes (Apple's own instructions). Save and the marks are part of the file. Of the three, this is the one I would reach for on a document I am reading alone.
Windows: Microsoft Edge. Edge is the default PDF reader on Windows and annotates in the tab that opened the file: highlight in four colours, freehand ink, erase, and add a text note (Microsoft's PDF reader documentation). Saving writes the marks into the PDF. No extension, no account, no download.
Either platform: Adobe Acrobat Reader. The free reader's comment tools cost nothing: sticky notes, highlight, strikethrough, freehand, shapes. What is paid is changing the PDF's own content, meaning the text, the page order, the export to Word. For annotation specifically the free tier is the whole job.
On an iPhone or iPad, open the PDF in Files or Books and tap the Markup icon. Same pen, same highlighter. This is the one situation where a phone is genuinely good at it.
Every route above ends in a file. Fine until someone else has to respond, and then it is the whole problem.
You annotate a copy. They annotate a different copy. Now there are two, and merging them is manual labour. A comment cannot be replied to, so the answer to "why is this figure wrong?" arrives in an email, detached from the page it was about. Nothing is live, so you find out what they think when the file comes back rather than while they are reading. And the version sitting in someone's Downloads folder is a fork that stays a fork.
None of this is a flaw in Preview or Edge. A PDF is a document format. It was never meant to carry a conversation, and asking it to is the mistake.
The fix is to stop sending the PDF and start sending a link to it with a layer on top.
One copy of the review exists and everybody is looking at it, because the annotations belong to the link rather than to the file. The same flow works for a hosted PDF you never downloaded and for one you dragged off your desktop.
| What you're doing | Use |
|---|---|
| Reading a paper and marking it up for yourself | Preview, Edge or Acrobat Reader. The marks save into the file |
| Handing in one annotated PDF as a deliverable | Preview, Edge or Acrobat Reader, same reason |
| Getting a client or a colleague to comment on a PDF | A link-based annotator, so they can reply without an account |
| Reviewing a hosted PDF you would rather not download | Paste its URL into a web annotator |
| Marking up a scan with no text layer | Anything with a pen. Text highlighting will not work anywhere |
Worth knowing before you pick it, because it decides whether this is the right tool:
No annotated file comes out. The marks are a layer over the PDF, not edits to it, so there is nothing to download and attach.
Three tools switch off. The element inspector, the measure tool and multi-select all read a page's HTML, and a PDF has none, so pointing at one would return the whole page rather than a heading or a button. They are hidden rather than left there to disappoint you.
A scanned PDF takes no text highlights, for the reason above. Everything else still works on it.
Uploads cap at 25MB, and an uploaded file is deleted 90 days after the link was last opened.
Annotating needs a pointer, so it is desktop only. Reading a link someone sent you works anywhere.
A PDF and an image go through identical machinery, because an image is a document of one page. Paste a link to a PNG, JPEG, GIF, WebP or AVIF, or upload one, and the drawing, comment and area tools behave the same way. That is covered in how to annotate an image.
Paste any URL at marklayer.app and annotate the live page in your browser. No install, no account, no sign-up. Recipients of your share links don't need to install anything either.
Annotate a page