How to Annotate a PDF: Highlights, Comments, and Stamps Explained

Key takeaways

• PDF annotations are a separate layer — they don't change the underlying document text
• Highlights mark what's important; sticky note comments explain why
• Free-form drawing tools (ink annotations) are best for diagrams, not text
• Flatten only when you're done — flattening merges annotations into the page permanently
• Browser-based annotators don't require app installs and keep your documents local

Annotating a PDF is one of the most common document tasks — and one of the most frequently misunderstood. Most people know they can highlight text, but fewer understand how the annotation layer works, what the different annotation types are for, or when flattening a document makes sense versus when to avoid it.

This guide covers the practical mechanics of PDF annotation without assumptions about your tool of choice.

How annotations work in a PDF

PDF annotations live in a separate layer from the page content. The underlying text, images, and layout of the document don’t change when you add a highlight or a comment. The annotation layer is a set of instructions that tells PDF viewers what to draw on top of the existing content.

This matters for a few reasons:

Non-destructive by default. Adding annotations doesn’t modify the source document. You can remove a highlight or change a comment without touching the original text.

Viewer-dependent rendering. Annotations are rendered by whatever PDF viewer opens the file. Most major viewers (Preview, Acrobat, PDF.js-based browser tools) render standard annotation types consistently, but some edge cases — complex freeform drawings, form-field annotations — may look different in different applications.

Easily stripped. Because annotations are separate from the page, they can be removed in bulk without affecting the document. This is useful for review workflows where you want to deliver a clean final copy.

The main annotation types and when to use them

Text highlight

The most common annotation. Selects a range of text and overlays a color band. Best used sparingly — over-highlighting removes the visual signal that makes highlights useful.

Use highlights for: key claims, definitions, numbers you’ll need to reference, sentences you want to come back to.

Sticky note / comment

A comment anchored to a specific point or selection in the document. Doesn’t mark the text itself — just places a callout icon that expands to show the note.

Use sticky notes for: your reasoning about a passage, questions you want to raise in review, flags that need follow-up. The comment is where you explain why something matters, not just that it does.

Text box

A free-floating text annotation you place anywhere on the page. Useful for adding information that isn’t tied to a specific word or passage — a summary of a section, a calculation result, a note about context.

Underline, strikethrough, and squiggly line

Markup annotations that apply directly to text. Underline for emphasis, strikethrough for suggesting deletion in review, squiggly underline (commonly used in document review tools) for flagging errors or uncertain content.

Free-form drawing / ink annotation

A vector drawing layer — the equivalent of drawing with a pen. Useful for marking up diagrams, circling figures, drawing arrows, or annotating content that isn’t text.

Ink annotations don’t interact with text selection, so they’re not useful for marking specific words or sentences. For text, use text markup annotations instead.

Stamp

A predefined or custom image overlay — “Approved,” “Draft,” “Confidential,” or any image you supply. Useful in document workflows that require a visual status marker.

Annotating without permanently changing the document

A common mistake is confusing annotating a PDF with editing it. Editing modifies the underlying content — changing text, moving elements, removing pages. Annotation adds a layer on top.

The safest practice for annotation is:

1. Work on a copy if the source document matters.
2. Don’t flatten until you’re certain the annotations are final (see below).
3. Save the annotated version separately from the clean original.

For most review and study purposes, you’ll never need to edit the underlying document at all. Annotations are sufficient.

Flattening: when and why

Flattening a PDF merges the annotation layer into the page content, making the annotations part of the document itself. Once flattened:

• The annotations can no longer be edited or removed as annotations
• The document looks the same in every viewer, including viewers that don’t fully support annotations
• The file size often increases slightly because the visual elements are now rasterized into the page

When to flatten:

• Before printing, if your printer doesn’t render annotations reliably
• For archiving a final annotated version where edits aren’t expected
• When sharing with someone whose viewer may not support annotations

When not to flatten:

• During active review — you lose the ability to change comments
• When you might need to remove annotations later
• When you want to keep annotations as a separate metadata layer for searchability

Practical annotation for review workflows

For document review — legal, academic, editorial — a structured approach helps:

First pass: mark without judging. Highlight everything that seems important. Use a single color.

Second pass: add comments. Go back to your highlights and add a sticky note to each one explaining what’s notable about it.

Third pass: prioritize. Use a second highlight color for the most critical passages. Some annotators support multiple highlight colors with distinct meanings.

Final pass: cleanup. Remove highlights that no longer seem important after reviewing. Resolve any questions you’ve flagged.

This keeps annotation from becoming a layer of visual noise that obscures the document rather than clarifying it.

Browser-based annotation

Modern browser-based PDF tools handle annotation without requiring a desktop application. Files stay local to your browser — no upload, no server processing. The annotation layer is written into the downloaded file when you save.

PDF Pilot supports highlight, comment, and text box annotations entirely in-browser. If you’re working on a document that shouldn’t leave your machine, this is the architecture to look for: client-side rendering with no network requests for the document content.