How to Insert Comments in a PDF File: Methods, Tools, and What to Consider

Adding comments to a PDF is one of the most common tasks in any document-heavy workflow — whether you're reviewing a contract, marking up a research paper, or collaborating on a design brief. The process is straightforward in principle, but the method you use depends heavily on what tools you have available, what device you're working on, and what kind of commenting you actually need to do.

What "Inserting a Comment" in a PDF Actually Means

PDF comments are annotations — layers of markup added on top of the document without altering the original text. They're stored as metadata within the file and can be shown, hidden, filtered, or exported separately from the document content.

There are several distinct types of PDF comments:

• Sticky notes — floating text boxes attached to a point in the document
• Text highlights — color-coded emphasis on selected text
• Strikethrough and underline — editorial markup for suggested changes
• Text boxes — free-floating typed annotations placed anywhere on the page
• Drawing tools — freehand shapes, arrows, and lines
• Stamps — pre-built or custom image overlays (e.g., "Approved," "Draft")

Not every tool supports all of these, and not every PDF is set up to accept them.

The Annotation Permission Factor

Before anything else: some PDFs are permission-locked. Document creators can restrict annotation in a PDF's security settings. If you try to comment on a restricted file, most tools will either block the action entirely or display a warning.

If a PDF is locked against annotations, you'll either need the document owner to remove the restriction, or — if you have the password — unlock it through a tool like Adobe Acrobat Pro. This is worth checking before troubleshooting why comments won't stick.

How to Add Comments Using Common Tools

Adobe Acrobat Reader (Free)

The free version of Acrobat Reader supports a limited but useful set of annotation tools — sticky notes, highlighting, and basic drawing. You'll find these under the Comment toolbar, which appears in the right-hand panel or via the Tools menu.

Adobe Acrobat Pro (paid) expands this significantly, adding stamps, measurement tools, the ability to edit comment formatting, and export summaries of all annotations.

Adobe Acrobat in a Browser (Acrobat Online)

Adobe offers a web-based version of Acrobat that allows basic commenting without installing software. You upload the file, use the annotation tools in the browser, and download the marked-up version. Useful for occasional use, but session-based and dependent on upload/download cycles.

macOS Preview

On a Mac, the built-in Preview app handles PDF annotation natively. Open a PDF, click the Markup Toolbar (the toolbox icon), and you'll have access to highlights, text notes, shapes, signatures, and freehand drawing. No additional software required.

Microsoft Edge and Chrome (Built-in PDF Viewers)

Both browsers include PDF viewers with basic annotation. Microsoft Edge is more capable here — it supports highlights, notes, and drawing tools directly in the browser. Chrome's PDF viewer has historically been more limited, though capabilities have expanded with updates. These are good for quick, low-friction comments but lack the depth of dedicated PDF tools.

Third-Party PDF Editors 🖊️

A wide ecosystem of apps offers PDF commenting, including:

• Foxit PDF Reader/Editor — robust annotation toolkit, available on Windows, macOS, and mobile
• PDF-XChange Editor — Windows-focused, with an extensive free tier for annotations
• Nitro PDF — business-oriented, with collaboration features
• Smallpdf, ILovePDF — browser-based tools for simple annotation tasks

Mobile: iOS and Android

On iOS, the Files app and built-in markup tools let you annotate PDFs without a third-party app. Many apps like Adobe Acrobat Mobile, GoodNotes, or Notability add more sophisticated pen-based commenting, useful on iPad with Apple Pencil.

On Android, Adobe Acrobat Mobile is the most capable free option. Samsung devices have additional native markup tools in their PDF viewer.

Comparing Annotation Approaches 📋

Collaboration and Comment Management

If multiple people are reviewing the same PDF, comment management becomes a real concern. Tools like Acrobat Pro and Foxit Business let you:

• Reply to comments in threaded conversations
• Resolve or mark comments as addressed
• Export comment summaries as a separate document
• Filter comments by author, type, or status

For lightweight collaboration, some teams share annotated PDFs by email, merging feedback manually. For ongoing review workflows, cloud-connected tools (Adobe Document Cloud, Dropbox with Acrobat integration) allow real-time shared annotation.

Variables That Affect Your Experience

Several factors shape which approach works best:

• Operating system — macOS, Windows, iOS, and Android each have different native capabilities
• File size and complexity — large PDFs with embedded fonts or complex layouts can behave differently across tools
• Whether the PDF is form-based or scanned — scanned image PDFs sometimes limit where text-anchored comments can be placed
• Who receives the file — if a colleague opens your annotated PDF in a different tool, some comment types may not render correctly
• Frequency of use — occasional annotation needs are well-served by free tools; heavy review workflows may justify a paid license

The right method also depends on whether you need comments to be visible to others in their PDF reader, or whether you're marking up for personal reference only. Some tools embed annotations in ways that are universally readable; others use proprietary formats that only display correctly in the same application.

How well any of these options fits your actual workflow comes down to the specific combination of your platform, how the PDF was created, who you're sharing it with, and how much annotation depth your work actually requires.