Best Free Online PDF Annotation Tools in 2026
Look, I've been there. You need to mark up a PDF — maybe you're reviewing a contract, annotating a report, or just highlighting important sections in an ebook — but you don't want to pay for Adobe Acrobat or install heavy software.
Good news: there are some genuinely solid free online tools that let you annotate PDFs without spending a dime. Let me walk you through the best ones.
What Can You Actually Do With PDF Annotation?
Before we get to the tools, let's talk about what "annotation" actually means. Most people think it's just highlighting, but there's a lot more you can do:
- Highlighting — Mark important text in different colors
- Underlining — Draw lines under key passages
- Strikethroughs — Cross out text you want to remove
- Comments/Sticky Notes — Add notes and feedback
- Freehand Drawing — Draw boxes, circles, or freeform shapes
- Text Boxes — Insert new text directly on the page
- Stamps — Add "Approved," "Draft," or custom stamps
The best part? Most online tools handle all of this in your browser. No installation needed.
Top Free Online PDF Annotation Tools
1. PDF24 Tools
PDF24's annotation tool is surprisingly powerful for something free. You get highlighting, underlining, stamps, freehand drawing, and text boxes. The interface is clean, and it processes files locally (better for privacy).
Best for: Anyone who wants a simple, no-signup-required annotation experience.
2. Xodo
Xodo offers one of the most full-featured online annotation experiences. Highlighting, commenting, drawing, and even form filling — it's all there. The free version works well, though some advanced features require an account.
Best for: Users who need collaborative features and don't mind creating an account.
3. Kami
Kami started as an education-focused tool, and it shows. It's fantastic for students and teachers, with integration with Google Classroom and other learning platforms. The annotation tools are robust, and there's a generous free tier.
Best for: Students, teachers, and anyone in education.
4. Adobe Online PDF Viewer
Adobe's free online viewer won't let you save annotated PDFs without paying, but you can still add comments and highlights — then print to PDF to save your annotations. It's a workaround, but it works in a pinch.
Best for: Quick reviews when you just need to markup and print.
5. SmallPDF
SmallPDF has an annotation tool alongside their other PDF utilities. It's not as feature-rich as some others, but the interface is super intuitive. Great for quick markups without a learning curve.
Best for: Quick, occasional annotation needs.
How to Annotate a PDF Online (Step-by-Step)
Here's the general process across most tools:
- Upload your PDF — Drag and drop or click to select
- Select your annotation tool — Usually in a toolbar on the left or top
- Apply annotations — Click and drag to highlight, click to add comments, etc.
- Save or download — Most tools let you download the annotated PDF
Pro tip: Some tools auto-save to their cloud, while others process locally. If privacy matters to you, look for tools that process files locally (like PDF24).
Comparing the Options
| Tool | Highlighting | Drawing | Comments | Free? |
|---|---|---|---|---|
| PDF24 | ✓ | ✓ | ✓ | ✓ |
| Xodo | ✓ | ✓ | ✓ | ✓ (with account) |
| Kami | ✓ | ✓ | ✓ | ✓ |
| SmallPDF | ✓ | ✗ | ✓ | ✓ |
What About Privacy?
This is important. When you upload a PDF to an online tool, you're trusting that company with your document. For sensitive business documents or personal files, this can be a real concern.
Here's my advice:
- Check the privacy policy — Do they keep or delete your files?
- Look for local processing — Tools that process in your browser are generally safer
- Avoid uploading highly sensitive documents — Contracts, medical records, legal stuff
PDF24 is one of the better options here — they explicitly state that files are processed locally and deleted after conversion.
When Online Tools Don't Cut It
If you need to annotate regularly, deal with large documents, or need offline access, consider these alternatives:
- PDF-XChange Editor — Free desktop editor with robust annotation
- LibreOffice Draw — Open source and surprisingly capable
- Okular — Great Linux option, also available on Windows
My Recommendation
For most people, PDF24 Tools is the way to go. It's free, privacy-focused, and handles all the basic annotation tasks without any hassle. No account needed, no upselling, just straightforward PDF annotation.
If you need collaboration features or work in education, Kami is worth exploring. And for power users who don't mind creating an account, Xodo offers the most features.