PDF Annotation Tool
Highlight passages, draw freehand, add arrows, shapes, hyperlinks, icon stamps, and signatures — all directly in your browser. Zoom in for precise annotation, pan with the hand tool, then download a single annotated PDF with everything permanently embedded. Your file never leaves your device.
How to Use the PDF Annotation Tool
Drop a PDF file onto the upload zone or click to browse. The first page renders immediately using PDF.js — no server involved. Thumbnail previews for all pages appear in the left sidebar.
Click any tool in the top toolbar — Highlight, Draw, Arrow, Rectangle, Text, Stamp, Sticky Note, and more. The right Properties panel shows relevant controls: color swatches, opacity slider, stroke width, font size, or stamp label depending on the active tool.
Click or drag on the PDF canvas to place annotations. Use the ‹ › page buttons or click thumbnails to move between pages. Annotations on each page are remembered. Use Select mode to reposition or delete any annotation. Ctrl+Z undoes the last action.
Click the green Download PDF button. pdf-lib embeds all annotations from every page into the original PDF structure and triggers a download. The output file is fully compatible with Adobe Acrobat, Preview, and all standard PDF viewers.
Features
Drag to mark text passages with color highlights, underlines, or strikethrough lines. Multiple colors available.
Pen tool for handwriting, arrows, circles, and underlines with adjustable color and stroke width.
Draw clean directional arrows and straight lines to point at specific content on the page.
Add rectangles and ellipses with optional fill color — useful for boxing content or drawing attention to areas.
Place typed text anywhere on the page with font size, color, bold/italic, and font family controls.
Add movable colored notes with editable text. Double-click any note to edit it. Embedded as colored blocks in download.
One-click placement of APPROVED, DRAFT, CONFIDENTIAL, FINAL, VOID, and other professional stamps in distinctive colors.
Stamp checkmarks or cross marks at any size — useful for reviewing forms, checklists, or approval workflows.
Insert any image (JPG, PNG, WebP) onto the page, then drag it to position. Embedded directly into the PDF download.
Every action is tracked. Ctrl+Z undoes, Ctrl+Y redoes. Move, edit, or delete operations are all undoable.
Thumbnail sidebar shows all pages. Click any thumbnail or use nav buttons to switch pages. All pages annotated on download.
PDF.js and pdf-lib run locally. No file is uploaded. Works offline after the page loads. No signup required.
What Is a PDF Annotation Tool?
A PDF annotation tool lets you mark up a PDF document — adding visual layers on top of the existing content without altering the underlying text or layout. You can highlight important passages, draw attention to diagrams, leave reviewer notes, approve sections with stamps, or add your own text comments. Annotations are stored as separate layers on the page, and when the PDF is saved, those layers are merged into the page content so any PDF viewer can display them.
This browser-based version uses PDF.js (Mozilla's open-source PDF renderer) to display your document and pdf-lib to write the annotations permanently into the PDF when you download. All processing runs inside your browser using WebAssembly and JavaScript — your file is never sent anywhere. It works the same way whether you are on Windows, macOS, Linux, or a Chromebook, and in any modern browser including Chrome, Firefox, Safari, and Edge.
Who Uses PDF Annotation Tools?
Annotation is central to how professionals review and communicate about documents across nearly every field. Students highlight key passages in research papers, mark chapters for revision, and draw diagrams in lecture slides before printing them. Researchers annotate academic papers with notes about relevant methods, questionable data, or sections to quote in their own writing. Teachers mark student assignments with corrections, highlight strong arguments, and add feedback arrows pointing to specific lines.
In professional settings, legal reviewers highlight specific contract clauses for negotiation, stamp documents as APPROVED or UNDER REVIEW, and draw lines connecting related sections. Medical professionals annotate clinical protocols and patient reports. Designers mark up PDF mockups with annotation arrows, text notes, and correction marks for handoff to developers. Project managers highlight deliverables in specification PDFs before team reviews. What all these users share is the need to mark up a document for communication purposes while keeping the original content completely intact — exactly what this tool enables.
How the Download Pipeline Works
When you click Download PDF, the tool loads your original PDF bytes through pdf-lib's PDFDocument.load() API. It then iterates through every page in order, collecting all annotations you placed on that page. For each annotation, it applies a coordinate transform: the on-screen annotation position in canvas pixels is divided by the page scale factor (1.5×) and the canvas-to-display ratio, converting back to PDF points — the unit PDF uses internally. The Y axis is also flipped, since PDF coordinates start at the bottom-left while canvas coordinates start at the top-left.
Annotations are drawn in two passes. The first pass renders all non-highlight annotations: shapes, arrows, text, stamps, sticky notes, and images. The second pass draws highlights on top, ensuring they appear as transparent overlays above any other annotation rather than being covered by white rectangle fills. This matches the visual layering you see in the editor. The completed PDF bytes are then written to a Blob and triggered as a file download without any server round-trip.
Annotation Tool Comparison: This Tool vs PDF Editor
This PDF Annotation Tool and the PDF Editor Online serve different but complementary purposes. The Annotation Tool focuses on reviewing an existing PDF exactly as it is — you cannot change any original text or content, only add layers on top. This is the right choice for marking up documents for review, approvals, grading, or study notes.
The PDF Editor Online adds a unique capability: it can detect and temporarily remove existing text, let you replace it with corrected text in the same position, and then reconstruct the page with both the original layout and your correction. Use the Editor when you need to fix a typo, update a date, or correct a figure in a PDF where you no longer have the source document. Use this Annotation Tool when you need to mark up, comment on, or approve a PDF without touching its content. For filling in PDF forms specifically, the PDF Form Filler is purpose-built for detecting and filling form fields.
Tips for Effective PDF Annotation
Use colors as a system, not decoration. Before you start annotating a long document, decide what each color means — yellow for important facts, red for errors or concerns, green for approved sections, blue for questions. A consistent color scheme makes your annotations immediately readable to anyone reviewing the document after you.
Use arrows to create connections. When two sections of a document relate to each other — a figure caption and the figure itself, or a clause referencing another clause — drawing an arrow between them is more informative than highlighting both separately. The Arrow tool in this editor draws a directional line with an arrowhead, making the relationship explicit.
Layer sticky notes for detailed feedback. For longer comments that would not fit as inline text, sticky notes are ideal. They have an expandable text area that can hold several sentences of detail. Position the note near the relevant content, and in the downloaded PDF the full text is embedded as a readable block. For shorter annotations, the Text label tool is quicker to use.
Use stamps for workflow status. If you are part of a document review process — approving invoices, reviewing contracts, or signing off on specifications — the text stamp tool provides the most efficient single-click placement of APPROVED, REVIEWED, or DRAFT markers. These are especially useful for multi-page documents where status needs to be visible on the first page.
Common Annotation Scenarios and How to Handle Them
Marking up a research paper for study: Use Highlight (yellow) for key findings, Highlight (red) for claims you want to verify, and Sticky Notes for your own analysis or questions. Navigate through pages using the thumbnail sidebar and annotate each section as you read. When done, download the annotated PDF and use it as your primary study reference.
Contract review before signing: Use Highlight (yellow) for standard clauses you are fine with, Highlight (red) for clauses requiring negotiation, and sticky notes to record the specific concern next to each highlighted clause. For items you have agreed to remove, use the Strikethrough tool. For sections that need to be added, use the Text label tool to note the addition in context. Share the downloaded annotated PDF with your legal team or counterparty.
Providing design feedback: Open the PDF mockup, switch to the Arrow tool, and draw arrows pointing to specific UI elements with Text labels nearby explaining the requested change. Use Rectangle to box entire sections needing redesign. Sticky notes work well for longer explanations that would crowd the design if placed inline. The annotated PDF can then be directly shared with the designer as a clear, actionable brief.
Why Browser-Based Matters for Annotation
Desktop PDF annotation software like Adobe Acrobat costs $23/month or more per user. Alternative apps require installation, account creation, and often store your documents in their cloud — which is a concern for confidential legal, medical, or financial documents. This tool runs entirely in your browser with no installation, no account, and no cloud storage. The PDF file itself is processed in memory and never transmitted.
Browser-based processing also means you can annotate PDFs on any computer without installing anything — a coworker's machine, a library computer, or a tablet. The annotation session is contained within the browser tab, and closing the tab clears everything — your document and annotations are never left on the device's storage. For sensitive documents, this is often preferable to cloud-based alternatives that retain your files on their servers after processing.
Known Limitations
Freehand draw path in download: Freehand pen strokes are embedded as a series of short line segments connecting the points you drew. Very fast strokes may have fewer points captured due to pointer event sampling, which can result in slightly angular curves rather than perfectly smooth ones. For clean curved shapes, draw slowly and steadily.
Text in annotations vs. original text: Text labels you place are embedded using standard PDF fonts (Helvetica, Times, Courier). If the original PDF uses custom embedded fonts and you are trying to match its style, there will be a font difference. This tool does not support embedding custom fonts — for exact-match text replacement, the PDF Editor Online handles this by working with the original page text layer directly.
Very large PDFs: PDFs with 100+ pages and heavy graphics may take a few seconds to load. All pages are rendered to thumbnails on load, which consumes memory proportional to the page count. For documents above 200 pages, consider working in smaller splits using the PDF Splitter first.