Annotation is how a document becomes a conversation. You are not changing what the author wrote — you are adding a layer over it: what matters, what is wrong, what you want changed, what you agreed to. Done well it saves a meeting. Done badly it produces a document covered in colour that nobody can act on.
The annotation types and what each is for
Most tools offer the same set, and each has a job it does better than the others.
Highlight — "read this bit." Marks importance without saying anything. Its entire value is scarcity; a page half-covered in yellow communicates nothing at all.
Comment or sticky note — "here is what I think." The workhorse of review. A highlight says this; a comment says why. Anchored to a point in the text and usually collapsible so it does not obscure the page.
Strikethrough — "delete this." Unambiguous in a way that a comment saying "I'd cut this" is not.
Underline — softer than highlight, useful when you want to mark several things without turning the page into a rainbow.
Text box — a note placed directly on the page rather than attached to a point. Right for margin notes and for annotations that need to be visible without clicking.
Stamps — approved, rejected, a checkmark, a cross. Fast, unambiguous status marks. Especially good on forms and checklists.
Drawing and shapes — arrows and circles pointing at things that are not text: a figure, a layout problem, a region of a scanned page.
Two kinds of annotation, and why it matters
Under the surface there is an important distinction.
Text-anchored annotations know which characters they refer to. A highlight applied to selected text is bound to that text: it survives reflow on a phone, can be exported as a list of quotations, and is what a review tool means by a "comment". These require the PDF to contain real text.
Free-form annotations are shapes and boxes placed at coordinates. They know nothing about what is underneath. They work on anything, including scans, but cannot be extracted as structured feedback.
If you can select text in the document, you can use the first kind. If you cannot, the page is an image and everything you add is the second kind.
Doing it in the browser
For marking up a document to send back, Docento.app covers the common ground: text boxes for comments and margin notes, stamps for checkmarks and crosses, and signatures or uploaded images where you need them. Everything is placed on the page and merged into the document on export, so recipients see your markup in any viewer without special software — and the file is processed on your own machine rather than uploaded.
Being straight about the limits: there is no dedicated highlighter, no drawing tool, and no shape tool, so true text highlights and freehand arrows need something else. For reading and studying — where you want a highlighter, a highlights sidebar, and text-anchored marks — the better tools are the ones you already have: macOS Preview (Cmd-Ctrl-H to highlight, with a highlights sidebar), Adobe Acrobat Reader, which is free and has the full annotation set including a comment list, and the built-in viewers in Chrome, Edge, and Firefox, all of which now annotate and save. On an iPad with a Pencil, Markup in Files or Books is the most pleasant annotation experience on any platform.
Making annotations people can act on
This is where most document review actually fails, and it has nothing to do with tooling.
Say what you want to happen. "Unclear" tells the author there is a problem and nothing about the solution. "Unclear whether this covers subcontractors — can we say explicitly?" can be acted on immediately. The test for every comment: could the recipient act on this without asking you a follow-up question?
Separate opinion from instruction. "This must change before signing" and "I'd probably phrase this differently" require completely different responses, and a page of undifferentiated comments makes the author guess which is which. Mark blocking issues distinctly.
Do not highlight everything. The most common failure. If a third of the page is marked, the marks carry no information. Highlight the sentence, not the paragraph.
One comment per issue. Three separate points inside one long note means two of them get missed.
Use colour consistently, and do not rely on it. A scheme — red for blocking, yellow for questions, green for approved — works well, provided you apply it the same way every time and include a key on the first page. But roughly 1 in 12 men has some colour vision deficiency, and red-green is precisely the pair most affected, so never let colour be the only signal. Add a word.
Sign your comments when several reviewers are marking the same document. "Who wrote this?" wastes more time than typing your initials.
Clean up before sending. Your private reading notes are not review feedback. Remove them.
Sharing marked-up documents
Once exported, annotations are part of the file and every recipient sees them in any viewer — no special software, no compatibility questions. That is the advantage of flattening markup into the page.
The trade-off is that flattened comments cannot be replied to or resolved. If the document is going through several rounds of review with multiple people, a tool with a proper comment thread — Acrobat, or a document platform with review features — beats emailing annotated copies back and forth, where the third round inevitably produces two divergent versions.
For heavily annotated scans, the file may grow enough to bounce off a mail server; compressing it usually solves that.
Related: how to highlight text in a PDF, how to flatten a PDF, and the beginners guide to editing PDFs.
