Marking up a PDF is the daily work of reviewing: flag the clause, circle the number, leave a note for the author. Done right, those marks travel inside the file itself — visible to every reader, editable by every reviewer, and completely separate from the document's actual content. Here's how the annotation layer works and how to use it without an account or an upload.
It means adding markup that lives beside the content, not in it. The PDF specification defines a family of annotation objects — each one is a small dictionary attached to a page, carrying its position, appearance, and often its own author and timestamp. The page's real content (text, fonts, images) never changes; the viewer draws the annotations on top. That separation is what makes annotation reversible, which is exactly what you want during review and exactly what you don't want when something must disappear (that's redaction, a different tool entirely).
Practically, you'll use five kinds of markup, and each maps to a standard annotation type:
| Markup | PDF annotation | Typical use |
|---|---|---|
| Highlight | /Highlight with quad points | Flag passages to revisit |
| Strikethrough | /StrikeOut | Mark text for deletion |
| Freehand drawing | /Ink stroke list | Circles, arrows, signatures-as-sketch |
| Text box | /FreeText | Comments typed on the page |
| Sticky note | /Text with contents | Threaded remarks that collapse to an icon |
Each page holds an /Annots array, and every mark you make becomes an entry in it. A highlight isn't a yellow rectangle that happens to sit over text — it's a typed object with a subtype, a bounding rectangle, and a list of quad points: four corners per highlighted line, in page coordinates measured from the bottom-left corner in points (a US Letter page is 612 × 792 points). A freehand scribble is an /Ink annotation carrying the polyline of your stroke; a text box is /FreeText carrying the characters plus a default-appearance string that says which font and size to draw them in.
Because these are structured objects rather than pixels, viewers can do more than display them: Acrobat opens the comment pane and threads replies; Preview lets you drag a note to a new spot; a compliance process can count every annotation and list who made it. A screenshot of a highlighted page does none of that.
Yes — standard annotation types are among the oldest, most widely supported parts of the PDF spec. Adobe Acrobat and Reader, macOS Preview, the Chrome and Edge built-in viewers, Firefox, and Okular all render highlight, ink, free text, and note annotations, and most let the next person edit or delete them. The occasional minimal mobile viewer ignores annotations outright; the document underneath is unaffected either way.
One round-trip matters more than the rest: saving through an editor that rewrites the whole file. If a tool re-exports a PDF as images or "optimizes" it carelessly, annotation objects can be dropped. When a marked-up file matters, open the copy you were sent, verify the marks are still there, and only then archive it.
Flattening is fusing the annotation layer into the page: your highlight stops being a /Highlight object and becomes ordinary page graphics, like the document's own ink. After flattening, nobody can open, edit, or delete the mark — it's part of the page.
Plenty of "free PDF annotation" sites are server converters with a marker tool in front — your contract goes up, marks come back down. The private alternative is a browser-based tool: the page renders the PDF locally (pdf.js does the rendering, the way Firefox's own viewer works), captures your marks on the annotation layer, and writes real annotation objects into a downloaded copy. Our PDF annotate tool does exactly that — highlight, strike, draw, text, and sticky notes, saved as the standard types in the table above, with the file never leaving the machine.
The conversion that happens on save is worth understanding once. Drag the highlighter across two lines at a typical viewing zoom and the tool records the rectangles in screen coordinates, then converts them to page coordinates on export — each line becomes a quad, and the saved highlight carries one bounding rectangle plus two quads' worth of corner points. Type a note at 24-pixel size and it lands as a /FreeText whose appearance string reads something like /Helv 17.78 Tf 0 0 0 rg: 17.78-point Helvetica, black — the pixel size divided by the zoom factor. Those details are why the same mark looks right in every viewer.
Slow down here. An ink annotation of your signature is visually a signature, and for low-stakes internal approvals people use exactly that. But a legally binding electronic signature generally rests on intent, consent to sign electronically, and some evidence tying the mark to the signer and the moment — audit trails, certificates, or witness processes that a plain annotation doesn't carry. Dedicated signing platforms exist because those extras are the actual product. For marking up contracts during negotiation, annotate freely; for executing them, use a real signing workflow — and treat that distinction as process advice, not legal advice.
A few habits keep marked-up files sane. Date your notes or use the sticky-note type so the comment pane shows who said what. Don't rely on color alone if a mark matters — write the word. When the review is done and the file needs restructuring, use a page organizer rather than annotating around the problem; our Organize PDF tool reorders, deletes, and rotates pages in the same private, in-browser way. And keep the vocabulary straight: annotate to discuss, flatten to finalize, redact to remove.
Open any PDF, highlight, strike through, draw, and add notes — then download a copy carrying real, editable PDF annotations. Free and fully in-browser.
PDF Annotate →Annotations are objects, not decoration: five standard types, understood by every serious viewer, separable from the content they sit on. Use them for review, flatten them only to finalize, and keep them far away from any job that calls for redaction. And when the document is confidential, annotate where it can't be uploaded — the browser.