How to Annotate a PDF for Free

Adding annotations to a PDF — highlights, comments, notes, freehand drawings — is a common workflow for reading, editing, and reviewing. Adobe Acrobat Pro is the traditional tool but costs $20/month. Free alternatives handle this fine.

On Mac: Preview (built-in)

Preview, the default PDF viewer on Mac, has full annotation tools:

1. Open PDF in Preview
2. View → Show Markup Toolbar (or press the markup icon)
3. Available tools:
   • Highlight: select text, highlight in color
   • Underline / Strikethrough: line under or through text
   • Add Text: insert text boxes anywhere
   • Add Note: sticky-note style comments
   • Shapes: rectangles, circles, lines, arrows
   • Freehand drawing (with mouse, trackpad, or Apple Pencil)
   • Signature: capture and insert signature
4. File → Save when done

Preview’s annotation tools are surprisingly capable. For most “review and mark up this PDF” tasks, Preview is enough.

On iPhone/iPad: built-in markup

iOS PDFs (in Files, Mail, Messages) all have annotation support:

1. Open the PDF
2. Tap the Markup icon (pencil tip, top right)
3. Use:
   • Marker / Pen / Pencil tools for drawing
   • Eraser
   • Color picker
   • Highlighter
   • Add Text feature
4. Save or share when done

With Apple Pencil on iPad: handwriting, sketching, marking up feels natural. For tablet annotation, iPad + Pencil + built-in markup is a great workflow.

On Windows: built-in Edge browser

Microsoft Edge’s PDF viewer has annotation:

1. Open PDF in Edge (most PDFs open by default in Edge)
2. Click annotation tools in the toolbar:
   • Highlight
   • Draw
   • Add comments
3. Click Save to keep changes

For more advanced features, install:

Adobe Reader (free, not Pro): basic annotation, viewing.

Foxit Reader (free): more annotation features than Adobe Reader.

Xodo (free): PDF reader with strong annotation.

On Android: Drive PDF viewer or apps

Google Drive’s PDF viewer: open PDF → tap pencil icon → markup tools available.

Free annotation apps: Xodo, Foxit, others on Play Store. Most have good annotation features in free tiers.

Cross-platform: free apps and browser tools

Xodo (free, cross-platform): full PDF editor with annotation, signatures, forms.

PDFsam Basic (free, desktop): not focused on annotation but useful for the broader PDF workflow.

Smallpdf, ilovepdf (online): limited free annotation features. Uploads required (privacy concern for sensitive PDFs).

For most users, the built-in tools above are sufficient.

What annotations actually do

Annotations are stored as part of the PDF — separate from the original page content. So:

• Original page is unchanged: the annotations are “on top” rather than replacing content
• Annotations are editable: you can move, delete, or modify them later
• They’re visible to anyone viewing the PDF: in any PDF reader
• They can be deleted without affecting the underlying document

This separation is useful: a reviewer can mark up a PDF with comments, the original document author can see exactly what was annotated.

Highlighting workflow

Common review workflow:

1. Open the PDF
2. Highlight key passages while reading
3. Add notes/comments at specific spots
4. Save the annotated version
5. Optionally combine with cover note: use PDF Merger to add a summary first page

Drawing and signatures

For signing or drawing:

Mac Preview: Camera signature (signs paper, photographs with webcam) or trackpad signature (draw with finger or stylus).

iPad with Apple Pencil: natural handwriting and signing.

Other platforms: app-specific tools.

For legal signatures: consider digital signature services (Adobe Sign, DocuSign) which provide audit trails and stronger validity than freehand signatures.

Removing annotations

If you receive a PDF with annotations and want to remove them:

Mac Preview: select annotation → Delete key.

Adobe Reader: similar select-and-delete.

To remove all annotations programmatically: tools like pdftk or qpdf can do this.

Flattening annotations

Sometimes you want to “burn” annotations into the PDF so they can’t be edited:

Adobe Acrobat Pro: Print to PDF — the resulting PDF has annotations as part of the page content, not separate objects.

Free alternative: print to PDF using your OS’s print dialog. Same effect.

Preview: doesn’t directly flatten; “Save As” preserves annotations as editable objects.

Comparing review approaches

For collaborative document review:

PDF annotations: each reviewer adds their marks; the document author sees all comments together.

Track changes in Word: more granular but requires the source format.

Modern tools (Hypothes.is, Notion comments): web-based collaboration with linked discussions.

For “review this final PDF”: PDF annotations are right. For “draft this document collaboratively”: use Word/Google Docs instead.

File size after annotation

Annotations are small. Adding highlights, comments, drawings to a 5 MB PDF typically results in 5.1 MB. Negligible growth.

If size grew significantly: you might have inserted images or other media as part of annotations. Standard text/highlight annotations are very lightweight.

Privacy

Built-in tools (Preview, iOS markup, Edge) run locally. Annotations are added to your local PDF file. No upload, no third party.

Web-based annotation tools (Smallpdf, etc.) upload the PDF for processing. Privacy concern for sensitive content.

TL;DR

• Mac: Preview has full annotation tools (View → Show Markup Toolbar)
• iPhone/iPad: built-in Markup feature in Files/Mail
• Windows: Microsoft Edge PDF viewer has built-in annotation
• Android: Google Drive PDF viewer + free apps like Xodo
• Cross-platform free: Xodo, Foxit Reader
• For signing: Mac Preview signature capture or iPad with Apple Pencil
• For collaboration: use PDF annotations for “review this final PDF” workflows
• All these are free; no need for Adobe Acrobat Pro for basic annotation