Annotate a PDF online: highlight, comment, draw

How to annotate a PDF online for free: the short answer

To annotate a PDF online without installing anything, open the file in the reader above, choose the Annotate tool, then select a passage to highlight or draw directly on the page. When your annotations are in place, click Apply, review the document, then save the annotated PDF. It is free, needs no account and runs entirely in your browser: the file is never uploaded to a server.

This guide explains what annotating a PDF actually means, the eight available tools and when to use each, the cases that cause trouble (scanned documents, protected files, tablets) and what to do with the document once it is annotated. For a specific task, dedicated guides go further: highlighting a PDF online, adding a comment to a PDF, reviewing and correcting a PDF as a team or annotating a PDF on an iPad or tablet.

What does annotating a PDF actually mean?

A PDF is designed to be fixed: its layout is the same on every screen and every printer. Annotating it therefore does not mean rewriting it, but adding a layer of visual remarks on top: a highlight, a line, a note, a stamp. The original text stays intact underneath.

The PDF standard (ISO 32000) defines dedicated objects for this, called annotations. Each one has a type (highlight, underline, strikeout, ink, square, free text, note, stamp), a position on the page, a colour and, where relevant, text content. This standard format is what Certyneo Reader writes into your file.

In practice, your annotations are not an image pasted onto the page. Adobe Acrobat, Preview on Mac, the readers built into web browsers and most PDF apps recognise them as real annotations. Your recipient can display them, list them, reply to them in their own software or delete them one by one.

The annotation tools, and when to use each one

The Annotate tool brings together eight actions. The first three work on a text selection; the other five can be placed anywhere on the page. All of them offer five colours: yellow, green, blue, red and black.

Highlight, underline, strike out

Select a passage as you would on a web page: the highlight, underline or strikeout follows exactly the lines you chose, even across several lines. Highlighting marks what matters, underlining flags a point to check, and strikeout proposes a deletion. These three tools need selectable text: our guide to highlighting text in a PDF explains what to do when selection does not work.

Freehand drawing

The ink tool draws freely with a mouse, a finger or a stylus: circle a figure, connect two paragraphs, add an arrow or a bracket. It is the most expressive tool but also the least precise: keep it for visual cues and use the text tools for written remarks.

Framing an area

The frame draws a rectangle around a table, a signature, a logo or anything that is not selectable text. It draws the eye without hiding anything.

Text box and pinned comment

A text box writes your remark directly on the page, visible to everyone and printed with it. A pinned comment adds a small icon instead: its content appears when the reader clicks it or opens the comments panel of their software. The first suits short instructions that must stand out, the second longer explanations that should not clutter the page. To choose between them and write useful remarks, read our guide to commenting on a PDF.

Image stamp

The stamp places a PNG or JPG image on the page: a logo, a company seal, a ready-made “Seen” or “Paid” mark. A scanned signature placed this way remains a plain image with no evidential value: to bind someone, you need an electronic signature.

Annotating a PDF without Adobe Acrobat

The free version of Adobe Acrobat Reader lets you highlight and comment, but it has to be installed, and the more advanced features require a subscription. Several alternatives exist, each with its own limits.

• The PDF viewers built into recent browsers (Microsoft Edge, Firefox) offer a few highlighting, drawing or text tools, and the list varies between browsers and versions.
• Preview on Mac and the markup tool in the Files app on iPhone and iPad are complete, but tied to the Apple ecosystem.
• Many online annotation sites start by uploading your file to their servers, sometimes with a daily page or file limit, and ask for an account to save the result.
• Certyneo Reader works in any recent browser, on desktop and mobile, with no account and no limit, and processes the file locally.

Whatever tool you pick, check one thing first: annotations must be written in the standard PDF format. Some tools flatten everything into an image when saving; your recipient can then no longer select the text or remove a remark.

Annotating a scanned PDF: the OCR step

To a PDF reader, a scanned or photographed document is just an image: it contains no text. You can draw on it and add a frame, a text box, a comment or a stamp, but you cannot select a sentence to highlight or strike it out.

The fix is to make the scan searchable with OCR before annotating it. Character recognition adds an invisible text layer beneath the image, aligned with the words. Once this is done, selection works and the highlighting tools follow the lines of the document. Searching for a word in the PDF becomes possible at the same time.

Tip: if a PDF seems to contain text but the selection jumps from one block to another, it is often a document laid out in columns. Select one column at a time, starting from the beginning of the sentence.

Annotate, edit, redact: three different actions

These three operations are often confused, yet they have neither the same effect nor the same risks.

• Annotating adds remarks on top of the content without changing it. The original text stays readable and copyable, and each annotation can be removed.
• Editing changes the content itself: fixing a date, a typo or an amount directly in the line. That is the job of the tool to edit the text of a PDF, which retypes the line in a font with the same metrics.
• Redacting permanently removes information: the hidden text is deleted from the file, not just covered. For a social security number, an IBAN or a name to anonymise, use the tool to redact a PDF.

The classic trap is drawing a black rectangle over sensitive data and believing it is gone. A frame or a drawing is still an annotation: the text underneath still exists, and anyone can copy it or remove the annotation. To hide information, redact it; to flag it, annotate it.

Reviewing a document as a team

Annotation is the common language of review: a report, a thesis, a mock-up or a contract circulates as a PDF, everyone adds their remarks, and the author carries them over into the source document. For this to work, agree on conventions before starting: one colour per reviewer, strikeout for deletions, underline for points to check, a text box for proposed replacements.

When several reviewers each work on their own copy, it becomes hard to tell what changed between versions. The tool to compare two versions of a PDF highlights added and removed lines. Our full method for reviewing and correcting a PDF as a team covers conventions, the review cycle and the special case of a contract before signing.

Annotating a PDF on an iPad, tablet or phone

The tool works in your tablet or phone browser, with no app to install. On a touchscreen, drawing follows your finger or stylus, and text is selected with a long press, as on any web page.

On a large screen, landscape mode and zoom make selection more precise. The annotated file is saved to the device's downloads folder. Our guide to annotating a PDF on an iPad or Android tablet gives step-by-step instructions and useful settings for writing with a stylus.

Privacy: why annotation happens on your device

The documents people annotate are rarely trivial: contracts, payslips, medical records, exam papers, legal case files. Handing them to an online service that stores them, even temporarily, raises a real confidentiality question, and sometimes a GDPR compliance one.

Certyneo Reader reads and edits the PDF directly in your browser. The file is never uploaded: Certyneo does not receive it, does not keep it and cannot view it. Closing the tab clears the document from memory. This is also what makes it possible to annotate large files, up to 150 MB, with no upload time.

A password-protected PDF opens once you enter its password, which is checked only on your device. If the annotated document must be sent encrypted, use the tool to password-protect a PDF before sending it.

Apply, fix, save: how it works step by step

Until you click Apply, your annotations are pending: they are shown on the page but not yet written into the file. The counter in the toolbar shows how many there are.

1. To remove a pending annotation, choose the Remove tool and click it. The Discard all button clears every pending annotation at once.
2. Click Apply: all annotations are written into the PDF in a single operation, and the page reloads.
3. If the result is not right, the “Undo the last change” command restores the previous state, as long as the file has not been saved.
4. Click Save to download the annotated PDF. Until you do, the reader warns you before you leave the page.

Your annotations are added to those the document already contained, without erasing them. To delete an annotation that has already been saved, open the file in a reader that lists comments, such as Adobe Acrobat Reader or Preview: every standard annotation can be selected and deleted there.

After annotating: share, compare, get it signed

An annotated PDF is rarely the end of the road. Depending on the case, the next step is to send it back to its author so the remarks can be applied, to compress it for email, or to read it one last time in the online PDF reader before sharing it.

When the document is a contract, a quote or a purchase order, the last step is often the signature. Once the remarks are applied and the final version is settled, you can sign a PDF online with an electronic signature: your signers sign from their phone, and the signed document is sealed, so any later change cannot go unnoticed.

Good practices for readable annotations

• One idea per annotation: a comment mixing three remarks is rarely handled in full.
• A colour code agreed from the start, for example yellow for key information, red for an error, green for an approval.
• Remarks that suggest a fix rather than just an observation: “replace 30 days with 45 days” rather than “deadline to review”.
• Sparing use of highlighting: a fully highlighted paragraph no longer signals anything.
• A clear file name when saving, with the date and your initials, so everyone knows which version they are holding.
• A final read of the annotated PDF before sending it, especially to make sure no internal remark stays in the client's copy.