How to Add Comments to a PDF: Tools, Methods, and What to Know First

Adding comments to a PDF sounds simple — and often it is. But the right method depends on what software you're using, what device you're on, and what kind of commenting you actually need. Here's a clear breakdown of how PDF commenting works, what your options are, and what shapes the experience.

What "Commenting" Actually Means in a PDF

PDF commenting is a broad term that covers several distinct annotation types:

• Text comments (sticky notes): Pop-up notes attached to a specific point in the document
• Highlight, underline, or strikethrough: Markup applied directly to selected text
• Text boxes: Freestanding typed text placed anywhere on the page
• Drawing tools: Shapes, arrows, and freehand markup
• Stamps: Pre-built or custom labels like "Approved," "Draft," or "Confidential"
• Tracked changes / markup summaries: Used in review workflows where multiple people comment and respond

Not every tool supports every type. A lightweight PDF viewer might let you highlight text but won't support stamps or threaded replies. A full PDF editor handles the entire range.

How PDF Comments Are Stored

Comments in a PDF are stored as annotations — a layer that sits on top of the document content without modifying the underlying text or layout. This matters for a few reasons:

• Comments can be shown or hidden without altering the document
• They can be exported as a separate summary
• They may or may not be visible when printed, depending on print settings
• Some recipients won't see them if their viewer doesn't support annotation rendering

When you save a commented PDF, the annotations are embedded in the file. The original content stays intact unless you explicitly "flatten" the PDF, which bakes the annotations permanently into the page.

Adding Comments on Desktop

Adobe Acrobat (Windows and macOS)

Adobe Acrobat — both the paid Pro version and the free Reader (with limitations) — is the most full-featured option. In Acrobat Reader, you can add sticky notes, highlights, and basic drawing markup. In Acrobat Pro, you get the full annotation suite plus the ability to create comment summaries and manage review workflows.

To add a comment in Acrobat:

1. Open the PDF
2. Go to Tools > Comment (or click the comment icon in the toolbar)
3. Select your annotation type
4. Click or drag on the document to place it

PDF-XChange Editor (Windows)

A popular alternative on Windows, PDF-XChange supports a wide annotation toolkit and is known for being fast with large documents. Its commenting tools are accessible from the Comment tab in the ribbon.

Preview (macOS)

Apple's built-in Preview app handles basic commenting well — highlights, sticky notes, shapes, and freehand drawing are all available under the Markup toolbar. It's limited in review workflow features but covers everyday annotation needs without installing anything extra.

LibreOffice Draw / Foxit PDF Editor

Both support commenting to varying degrees. Foxit is often used in enterprise environments as an Acrobat alternative and supports full annotation features. LibreOffice Draw can open and annotate PDFs but is better suited for light use.

Adding Comments on Mobile 📱

iOS (iPhone and iPad)

The Files app and Books app both support basic PDF markup. For more control, Adobe Acrobat mobile and PDF Expert offer rich annotation tools. On iPad with Apple Pencil, freehand annotation becomes particularly fluid.

Android

Adobe Acrobat for Android covers the main annotation types. Xodo PDF is a well-regarded free option with a clean interface and solid markup support.

The gap between mobile and desktop annotation is mostly around precision and workflow — mobile tools are fine for quick comments and highlights, but managing a multi-page review with lots of annotations is generally easier on a larger screen.

Adding Comments Through a Browser

Most modern browsers can display PDFs inline, but annotation support varies:

For browser-based annotation beyond these limits, tools like Adobe Acrobat online, Smallpdf, or PDF.co let you upload a PDF, annotate in the browser, and download the result — no installation needed.

Key Variables That Affect Your Experience 🔍

1. PDF permissions Some PDFs are protected with restrictions that block annotation. If commenting tools appear grayed out, the document owner may have disabled editing or markup. You'd need either the document password or a tool that can work around those restrictions (where legally permitted).

2. PDF version and structure Scanned PDFs (image-based) behave differently from text-based PDFs. You can still place sticky notes or draw on a scanned PDF, but you can't highlight text because there's no selectable text layer — unless the file has had OCR (optical character recognition) applied.

3. Collaboration needs If you're the only one commenting, almost any tool works. If you need threaded replies, comment tracking across multiple reviewers, or integration with a document management system, the tool requirements shift significantly. Adobe Acrobat's shared review features, for example, are designed specifically for multi-reviewer workflows.

4. File size and performance Large PDFs — particularly those with many images or embedded assets — can slow down annotation tools on lower-powered devices. What runs smoothly on a modern laptop may lag on an older machine or a mid-range phone.

5. Output requirements If comments need to be printed, exported as a summary, or stripped out before final delivery, not every tool makes that equally easy. Acrobat Pro has the most granular control over comment export and flattening. Simpler tools may just embed the annotations with no easy way to manage them post-hoc.

The right setup depends entirely on what kind of document you're annotating, who else needs to see or respond to those comments, and what happens to the file afterward.