How to annotate a PDF (and find the annotation again)

You have highlighted forty PDFs. Somewhere in them is the paragraph you need. It was about sample sizes, in the paper with the blue figures, from some time in spring. You are fairly sure you highlighted it. You are not going to find it.

This is a strange problem to have, because the highlighting itself was easy. Every PDF reader on your machine can do it, and there are a dozen guides explaining how. What none of them explain is the part you are stuck on now.

So this article does both. First the mechanics, honestly: how to mark up a PDF in the three places you are most likely to be doing it. Then the part that decides whether any of it was worth the effort.

Three kinds of mark, and which one you actually want

Every PDF tool offers roughly the same three things, and people reach for the wrong one constantly.

A highlight is a colour over text. It marks where something is and says nothing about why. It is fast, it survives being reopened, and its one weakness is that a page of yellow is exactly as useless as a page of white.

A comment is text attached to a selection. It is the only one of the three that records why you stopped. “This contradicts Meyer” is worth more in six months than any amount of yellow.

A note is a comment that collapses into an icon, so it does not cover the page. Same content, smaller footprint.

The useful habit: highlight while reading, because stopping to type breaks the reading. Then, at the end of a session, go back and turn the highlights that mattered into comments. Most of them will not survive that pass, which is the point. The pass is where you decide what you actually thought.

Annotating in Preview on a Mac

Preview is already installed and it is better than its reputation. Apple’s official guide to annotating in Preview documents every tool on the markup toolbar. You need four of them.

Open the PDF and click the Show Markup Toolbar button to reveal the tools. Then:

• The Highlight tool lays colour across text as you drag over it. If you have already selected some text, Highlight Selection applies the colour to exactly that selection, which is the more precise of the two.
• The Note tool takes a click and then your typing. Control-click the note afterwards to change its colour. That is the cheapest colour-coding scheme there is: one colour for “method”, one for “disagree”, one for “cite this”.
• The Text tool gives you a free-floating text box, for when your comment belongs beside a figure rather than attached to a sentence.
• Rectangular Selection is not an annotation at all, but it is the fastest way to lift a figure out of a paper and into your notes.

The save step is the one that matters. Apple’s guide names two routes and they are not equivalent. File > Save, File > Export, or File > Export to PDF keeps your annotations as annotations, still selectable and still removable. File > Print, then Save as PDF, flattens them permanently: the highlight becomes part of the page image and the comment text becomes a picture of text.

Flattening is right when you are sending a marked-up contract to someone who must not be able to edit your marks. It is wrong, irreversibly, for anything you intend to read again.

Annotating in Acrobat

Acrobat Reader is free and handles annotation fine. You do not need a paid subscription to highlight, comment or add a sticky note, and the free tier will not stop you. Open the PDF, find the comment or markup toolbar, and the same three tools are there under slightly different names.

What the free tier does not give you is editing the document itself: changing the underlying text, reordering pages, building or flattening a form, running OCR on a scan. Filling in a form and signing it are free, which is worth saying because they are the two people most often assume are not. The rest sits behind the subscription. Adobe moves these lines around between releases, so check the current split before you rely on it. The distinction is worth knowing before you start, because “I need to fix a typo in this PDF” and “I need to leave a comment on this PDF” feel like the same task and are priced very differently.

Acrobat’s clearest advantage over Preview is its comments list, a panel that shows every comment in the document as a scrollable list, so you can review what you wrote without paging through it. Keep that panel in mind. It is the closest any of these tools comes to solving the problem at the end of this article, and noticing exactly how far short it falls is instructive.

Annotating in a browser, with nothing installed

Sometimes you are on a machine that is not yours. Browser annotators handle this: upload the file, mark it up, download it. Smallpdf’s annotator is representative, offering text, a pencil, a highlighter with colour choice, shapes, callouts, underline, strikethrough and arrows. You finish by downloading the annotated file or generating a share link.

The trade-off is stated plainly on the page: files are auto-deleted after one hour. For a borrowed machine that is a feature, because nothing of yours is left behind. For your own work it is a hard deadline. Download before the hour is up or the annotation is gone, and there is no version of “I will get back to it tomorrow” that survives this.

The deeper trade-off is that a browser tool has no idea what else you have read. Every session starts from nothing, because every session is nothing: one file, one hour, then deleted.

Where all of this stops

Notice what none of the three can do.

Your annotation lives inside the PDF file. That is the format working as designed, not an oversight somebody will get around to fixing. A PDF is a self-contained document, and an annotation is part of the document. Which means:

• You cannot search across it. Your reader searches the file that is open. Ask “which of my papers did I mark up about attrition?” and there is nothing to ask.
• You cannot get it out. The highlight is a coloured rectangle at coordinates on a page. There is no list of your highlights, and Apple’s guide, thorough as it is about making the mark, says nothing about retrieving it, because there is nothing to say.
• It is not attached to the citation. The file knows its own contents. It does not know it is Meyer et al. 2019, and it does not know it is the paper you cited in chapter three.

This is why even the good guides end where they do. The University of Wisconsin-Whitewater library’s guide to annotating PDFs sits inside a collection called Research, Citation, & Class Guides, written by librarians for researchers, and it teaches you to double-click the file and click Add a comment. Those are good instructions. They also stop at the file boundary, because that is where the tools stop.

The vendor comparisons stop in the same place. Drawboard’s roundup of annotation apps compares nine of them on pen pressure, platform, collaboration, pricing and G2 scores. Cross-document search does not appear. Citation managers do not appear. Nine products, and not one of them is competing on the thing that goes wrong at paper forty.

What “getting it back” actually requires

If the goal is finding your own thinking again, three things have to be true, and they are separate.

Search has to cross documents. The question you will ask is “where did I write attrition, in anything I have ever read”, and answering it requires the annotations to exist somewhere outside the individual files, in an index or a library that spans them.

The annotation has to be extractable. You should be able to get a list: every highlight and comment from a paper, as text, that you can paste into a draft. A mark that can only be viewed in place is a mark you have to go back to the place for.

It has to point at the citation. The moment you use a note, you need the reference. If the annotation and the bibliographic record live in different systems, you are copying between them by hand, every time, forever. That friction is what stops people annotating at all.

None of the three is exotic. They are simply not what a PDF editor is for. A PDF editor’s job is one document, and these are properties of a library. Reference managers exist because that is a different job — though most of them assume the job is filing rather than reading, which is a complaint for another day.

Annotating a literature, not a file

The shift worth making is smaller than it sounds. Decide where the annotation will live before you make it, rather than afterwards.

If you are marking up a contract, a form, or a paper you will read once and never revisit, mark the file. Preview is right there and it is enough. If you are reading something you will cite, the annotation should be created somewhere it can be searched, exported and linked to a reference. In practice that means opening the paper inside whatever holds your library, rather than in whatever opened it by default when you double-clicked.

The cost of the wrong choice is invisible at the time. It shows up at paper forty, and by then the fix is re-reading thirty-nine PDFs.

Before you annotate your next paper

Three questions, none of which is about which highlighter to use.

1. Will I need this again? If no, mark the file and move on. Not everything is a research artefact.
2. Where will the mark live? Inside the file, or somewhere that can see all my other files? Decide now, because moving it later means doing it again.
3. Can I get from this mark to the citation? If getting the reference means opening a different application and searching for the title, the annotation is already costing more than it returns.

Then go and highlight something. The mechanics were never the hard part.