PDF Annotation Tools Compared: Highlight, Note, Draw

Almost every comparison of PDF annotation tools lists the same features and skips the one distinction that decides which tool you should use. A highlight in a PDF can be two completely different things, and the difference determines whether anyone can reply to it, remove it, or even see that it is a comment.

Two kinds of annotation, and why it matters

The PDF format has a dedicated object type for markup. A real annotation is a separate /Annot dictionary attached to a page, carrying its own type, coordinates, colour, author name, timestamp and comment text. It sits alongside the page content rather than inside it. Acrobat, Apple's Preview, Xodo and most desktop readers write this kind.

The other approach draws the markup straight into the page's content stream. A highlight becomes a translucent rectangle painted onto the page. A note becomes a coloured box with text rendered into it. This is called flattening, and once the file is saved there is no annotation object left, only graphics.

Both produce a file that looks identical on screen. The consequences diverge sharply afterwards:

• Real annotations appear in a reader's comment pane as a reviewable list. Flattened markup does not appear anywhere, because to the reader it is part of the page.
• Real annotations can be deleted, moved or replied to by whoever opens the file next. Flattened markup is permanent.
• Real annotations can be extracted, exported to a summary, or stripped out in bulk. Flattened markup cannot.
• Real annotations can be silently hidden by a reader that is configured not to display comments. Flattened markup always shows, which is occasionally exactly what you want.

Decide which behaviour your task needs before you compare feature lists. If a colleague has to respond to your comments, you need the first kind. If you are marking up a document for your own reading, or stamping something that must not be removable, the second kind is fine and sometimes better.

What RaptorPDF's annotator does, exactly

Our annotation tool is the flattening kind, and it is worth stating that plainly because it is the single most important thing to know before you use it. Everything runs in the browser through pdf.js for rendering and pdf-lib for output, and on download each mark is drawn into the page: highlights as filled rectangles, boxes and ellipses as stroked outlines, freehand strokes as short line segments, notes as a coloured rectangle with wrapped text, and free text drawn directly onto the page.

The practical consequences, in the order you will bump into them:

• Highlights are dragged rectangles, not text selections. There is no text layer under the annotation canvas, so the highlight does not snap to a line of type the way Preview or Acrobat does. On a single-column page this barely matters. On a two-column academic paper, or where a sentence wraps mid-line, you will be drawing several rectangles by hand and they will not follow the ragged edge of the text.
• Highlight opacity is capped. The output clamps highlight transparency so the text underneath always stays readable, which means you cannot accidentally paint over a paragraph, and you also cannot create an opaque colour block with the highlighter. Use the redaction tool for that.
• Text uses the standard Helvetica fonts. These cover the Latin-1 range only. If you type Turkish, Greek, Cyrillic, Polish or Chinese characters into a note, they will not embed and the text will come out wrong or fail outright. This is a real limitation of the standard PDF base fonts, not something we have worked around yet.
• Undo history is bounded. The tool keeps the last sixty steps. That is plenty for a page or two of markup and not enough for an afternoon of work on a long document, so save intermediate versions if the session is long.
• No authorship, no threading, no replies. There is nowhere to put an author name, because there is no annotation object to carry one. Collaborative review is not something this tool does.

What it does well is the case where you want markup that stays put, on a document you would rather not upload anywhere. Nothing is sent to a server, so a contract you are reading through, a medical letter, or a draft under embargo never leaves the tab. The free plan allows three operations a day with files up to 50 MB, with no watermark and no account. Pro is $4.99 a month or $39.99 a year and lifts the file ceiling to 300 MB.

If you are reading research papers

This is the one use case where a general PDF editor is the wrong tool, and it is worth saying so even though it costs us the recommendation. A reference manager with a built-in reader keeps the annotation attached to the bibliographic record rather than to a loose file in a downloads folder. Six months later, when you are writing and need the paragraph you highlighted, the difference between "search my library's annotations" and "which of these 340 PDFs was it in" is the whole argument.

Zotero's reader stores annotations in the library database and can export them into a note per item, which is what makes it useful for a literature review rather than for reading one paper. Handwriting-first apps on a tablet are the other reasonable answer, for a different reason: writing a sentence out by hand is slower than highlighting it, and that slowness is the point if you are trying to remember the material. Neither of those things is a feature a browser PDF editor can provide.

A colour scheme that survives contact with a real document

Colour coding advice usually fails because it has too many categories. Four or five colours sounds manageable in an article and collapses by page thirty, when you are holding a rule in your head instead of reading. Two colours is the version that actually sticks:

• One colour for what the document says that you will need to quote or cite later.
• One colour for your own reaction to it: a doubt, a contradiction, a question for the author.

Add a third only when you have a concrete downstream need for it, such as a specific colour meaning "this clause needs a lawyer's eye" on a contract review. The test for whether a category is real is whether you would ever search for it on its own.

Redaction is not annotation, and the distinction is dangerous

A black rectangle drawn over a name looks like the name is gone. It is not. If the page was copied into the output document without being re-rendered, the text object is still in the file underneath the rectangle, and anyone can recover it by selecting the area and copying, or by running a text extraction tool over the PDF. This has produced real, public, embarrassing disclosures in court filings more than once.

RaptorPDF's redaction tool draws an opaque rectangle. It hides content from view and it does not remove the underlying text. If you need the text genuinely gone, the reliable approach with a drawing-based tool is to redact and then run the result through the compressor, which rasterises every page into an image and therefore destroys the text layer along with everything else. You lose selectable text across the whole document, which is a heavy price, and that is the honest trade. For routine redaction of legal material, software with a real redaction engine that removes the content objects is the correct purchase.

Choosing

• Someone has to reply to your comments. Use a tool that writes real annotation objects. Acrobat, Preview or Xodo.
• You are marking up a confidential document for yourself. A local tool. Preview if you are on a Mac and want text-snapped highlights, RaptorPDF if you are on Windows, Linux or a Chromebook and want nothing uploaded.
• You are building a literature review. A reference manager's reader, not a PDF editor.
• You want the markup to be permanent and unremovable. A flattening tool is the right choice here, and that includes ours.
• You need real redaction. Buy software that removes content objects. Do not rely on a black box from any drawing tool, including this one.