Six free annotation tools compared

Step by step — annotate a contract for client review

1. Open the PDF in your annotation tool (Acrobat Reader is the common cross-platform choice; Apple Preview is faster if you are on Mac and alone).
2. Highlight key clauses as you read. Pick a colour convention: yellow for "approved as written", orange for "needs discussion", red for "blocker". Consistency across documents compounds.
3. Add sticky notes with specific questions or proposed changes. Keep notes terse and structured: "Re §4.2 — clarify whether this includes third-party costs?".
4. Export comment summary. Acrobat Reader: Comments → Options → Summarize Comments. Result is a PDF listing every annotation in order, with page number and author — useful as a separate cover for the client to read first.
5. Send both files to the client. Annotated PDF + comment summary. Client can read the summary first to see the macro shape of feedback, then dive into the annotated PDF for context.

Related reading

• PDF Pro vs free tools: where annotation features land in the Pro/free divide.
• How to sign a PDF: signatures are a specific type of annotation.
• PDF for lawyers: annotation in legal document review.
• Compare PDFs: complementary to annotation for redline workflows.
• PDF tools for students: annotation for textbooks and lecture notes.
• Share PDFs securely: for sharing annotated documents back to recipients.
• ScoutMyTool Edit PDF: free browser-based annotation tool.

FAQ

What is the difference between "annotating" a PDF and "editing" it?

Annotations are a layer on top of the PDF content — highlights, sticky notes, drawings, comments. They are stored in the PDF as annotation objects (per ISO 32000-1 §12.5) but the underlying page content (the text and images) is not modified. Editing means changing the page content itself: replacing text, moving images, deleting lines. Annotations preserve the original document while letting collaborators mark it up; editing produces a different document. For collaborative review (legal markup, peer feedback, internal sign-off), annotate. For making the source document correct (typo fix, version update), edit the source then re-export.

Can annotations from one tool be read by another?

Yes — annotation objects are part of the PDF specification, so any compliant viewer reads them regardless of which tool created them. Acrobat Reader can read annotations made in Apple Preview, Xodo, Foxit, and ScoutMyTool, and vice versa. Two caveats. First, tool-specific extensions (Acrobat's proprietary stamps, Xodo's sync metadata) may render as plain annotations in other tools, losing the original styling. Second, free-form drawings made with Apple Pencil or stylus on iPad are stored as ink annotations; not all readers display ink with the same fidelity. For collaborative work where multiple tools will be used, stick to the common annotation types (highlight, sticky note, free text box) for maximum cross-tool compatibility.

Which tool is best for collaborative PDF review with comments and threads?

Adobe Acrobat Reader with the "Share for Review" feature (free, requires Acrobat ID). Each reviewer opens the same link, adds annotations, and the comments thread back into a single shared view. Xodo Connect offers similar functionality with a more lightweight account model. For one-off review where everyone is in the same office, Apple Preview's simpler annotation flow is sometimes enough — send the PDF, each reviewer annotates locally, sends back, you manually merge comments. For high-volume professional review (legal redlining, academic peer review), dedicated platforms (Adobe Acrobat DC Reviews, Kami, Scribbr) provide audit trail, reviewer identity verification, and structured comment workflows beyond simple annotation.

I annotate PDFs heavily on iPad with Apple Pencil. Which tool is best?

PDF Expert (Readdle) and GoodReader for serious iPad PDF annotation. Both are designed around Pencil-first workflows: pressure-sensitive ink, palm rejection, snap-to-text highlighting, fast switching between annotation tools. Apple Books and the built-in Files app handle simple Pencil annotation but slow down on complex documents. Notability and OneNote are tangential — designed for note-taking rather than document annotation, but support PDF import for hybrid workflows. For students, PDF Expert's annotated PDFs sync via iCloud across iPad and Mac, which compounds the workflow value. The paid versions are $20–$50 one-time and pay back across a semester of reading.

Are free PDF annotation tools privacy-safe?

Depends on the architecture. Desktop tools (Acrobat Reader, Preview, Foxit installed locally) process annotations locally; the PDF does not leave your machine. Browser-only tools (ScoutMyTool Edit PDF) also run locally in your browser tab. Cloud-syncing tools (Xodo with sync, Adobe Acrobat web, Kami) upload your PDF to vendor servers for storage and cross-device sync — appropriate for non-sensitive collaborative documents, less appropriate for confidential drafts. For privileged or confidential work (legal, medical, financial drafts), prefer local-only annotation; for collaborative public-facing work, cloud-sync is fine.

How do I extract just the annotations from a PDF for review?

Acrobat Reader: Comments panel → Options → Summarize Comments → "Comments only" — produces a PDF listing every annotation with its content, page number, and author. Apple Preview: no direct extraction; copy annotations manually. ScoutMyTool Edit PDF has an "Export annotations as JSON or CSV" feature for downstream processing. The extracted comment list is useful for compliance reviews ("which clauses did Legal flag?"), academic peer review ("what feedback did each reviewer give?"), and audit trail ("who annotated when?"). For high-volume review, scripting against pdf-lib or PyPDF gives full programmatic control over annotation extraction.

How do I "flatten" annotations so they cannot be moved or removed?

Flattening converts annotation objects into part of the page content stream — they become non-editable picture elements rather than removable annotations. Acrobat Pro: Tools → Print Production → Flatten. ScoutMyTool Edit PDF: "Flatten annotations" option in the export dialog. After flattening, the visual appearance is unchanged but the annotations cannot be selected, edited, or removed in any reader. Use this for final delivery PDFs where you want comments and signatures visible but immutable. Note: flattening is one-way; keep the un-flattened source if you may need to revise annotations later.

Citations

1. ISO 32000-1:2008 — "Document management — Portable document format" — §12.5 (Annotations) defining the annotation object model.
2. Adobe Acrobat — Reader feature documentation including comments and collaborative review.
3. Apple — Preview User Guide — annotation tools on macOS.
4. Foxit Software — Reader documentation and feature comparison with Acrobat.