How to Annotate a PDF Free (2026)

The Core Annotation Types

Highlights are the most-used annotation — a colored overlay over text that preserves readability. Different colors typically mean different things (yellow for attention, green for approved, pink for questions), though nothing in the PDF standard enforces this convention. Highlights travel with the document and are selectable/removable individually.

Sticky notes are icon-based comments that appear at a point on the page. Clicking the note opens the comment text. They're useful for longer feedback that would clutter the margin if written out. Sticky notes preserve author name and timestamp automatically, which matters for review workflows where you need to track who said what.

Drawing and shape tools (freehand pen, rectangle, arrow, line, ellipse) let you mark up visual content — circling a section, drawing attention to a specific element on a page, or adding schematic feedback to design files. These are heavier than text-based annotations and can make a PDF feel cluttered quickly; use them where a highlight or note wouldn't be visually clear enough.

Text annotations (typed comments placed directly on the page, strikethroughs, underlines, and insertion marks) are the review-editor's toolkit. Strikethroughs show what should be removed; insertion carets show where new text should go; typed notes explain the reasoning. Any decent copy-edit workflow uses all three.

Method 1: UDT PDF Annotator (Free, Browser-Based)

The UDT PDF Annotator supports all the core annotation types in the browser. Load a PDF, annotate with highlights, sticky notes, shapes, and typed text, and export the annotated version. Annotations are saved into the PDF itself (not as a separate layer file), so the recipient sees your markup when they open the document in any standard PDF viewer.

The workflow: drop your PDF in, pick an annotation tool from the toolbar, click or drag on the page to place it, type any associated text, and continue through the document. Annotations can be selected, moved, resized, and deleted individually. When done, download the annotated PDF — the original file is preserved in memory so you can always export again if you need to revise.

Because everything runs locally, there are no file size limits beyond what your browser memory handles, no forced cloud sync, and annotations don't get sent to any server. That's meaningful for reviewing contracts, personnel files, medical records, or anything sensitive — the annotated file never leaves your machine until you explicitly share it.

Method 2: Acrobat, Preview, and Browser Viewers

Acrobat Reader (free) handles annotation well and is the de-facto standard for document review in corporate environments. Tools → Comment gives you the full annotation toolkit. The advantage is universal compatibility; the drawback is that Reader's UI has gotten dense over the years and performs slowly on large documents.

macOS Preview is the sleeper hero of PDF annotation. Built into every Mac, it handles highlights, notes, shapes, signatures, and freehand drawing with a clean interface. For Mac users who annotate a few PDFs a week, it's often all you need.

Browser viewers (Chrome, Edge, Firefox built-in) have grown basic annotation in recent versions. Chrome's PDF viewer now supports highlights and text annotations directly. For quick markup without opening another app, these are perfectly fine. The trade-off is limited tool variety and occasional compatibility issues when the annotated file is opened in other viewers.

Sharing Annotated PDFs

Annotations are saved into the PDF file itself in standard formats, which means any modern PDF viewer (Acrobat, Preview, Foxit, browser viewers) will display them correctly. Send the annotated file by email or upload it to shared storage and the recipient sees your markup when they open it.

For collaborative review with multiple reviewers, the traditional pattern is "pass the file around" — each reviewer adds their annotations and passes to the next. This works but produces versioning headaches (who has the latest? whose annotations overwrote whose?). For structured review with many reviewers, dedicated document-review platforms (Adobe Acrobat Online, Kami, Hypothes.is) handle the merging automatically, at the cost of uploading your document to their cloud.

If the annotated PDF needs to be archived as a final "approved" version, flatten the annotations into the page content via the PDF flattener before archiving. Flattening makes annotations part of the page itself so they can't be accidentally deleted by anyone opening the file later.

Annotation Etiquette

For legal or contract review, stick to structured markup: strikethroughs for removals, inserts for additions, sticky notes for rationale. Freeform drawings confuse redline workflows and are harder for the other side to respond to. If the review is going to get passed to opposing counsel, match whatever convention their team uses (common ones: MS Word track changes exported as PDF, or Acrobat comments).

For design or visual review, the opposite is often true — drawings and shapes are clearer than text. A circle around an element with a short comment ("too saturated") communicates faster than a sticky note describing the location verbally. Match the annotation style to what your recipient will find actionable.

Don't annotate sensitive information directly into a sticky note. Annotations carry author names and timestamps in their metadata. If you write "I don't trust the opposing team's numbers here" in a sticky note on a contract draft you're sending to the opposing team, that comment is in the file they receive. Review your annotations for internal-only content before sending.