Someone sends you a contract to review, and you want to flag a couple of questionable clauses without rewriting the whole file. You open it in a browser, and it turns out there is nothing to highlight a phrase with: the text will not edit, you cannot write in the margins, and emailing your notes separately as "paragraph 3, line 2, replace the word" is awkward for everyone. It is a familiar spot. A PDF is meant to be the final "ready to print" format, so the usual commenting tools feel absent.
They are there, though. They are called annotations. By the standard, a PDF can store highlights, underlines, margin notes, boxes, and freehand drawings on a separate layer without touching the original text. The recipient sees exactly what you marked, right in the document instead of in a cover email. The key is the difference between editing text and commenting on it. An annotation changes nothing in the content. It just pins your note to a specific spot on the page.
Why is this easier than a back-and-forth thread? When several people review the same file, comments in email drift away from the current version fast. One person explains "top of page two" while another has already sent a corrected file, and the numbering no longer lines up. An annotation physically lives at the right point in the document, so nobody has to hunt for coordinates. And when there are many edits, color-coded marks show the scope at a glance: flip through the file and you can see where there are ten notes and where there is one.
How to add annotations to a PDF, step by step
1. Open Annotate PDF and upload your document. The file loads with all its pages, so scroll to the spot you need. 2. Pick the tool for the job: a highlighter for a phrase, an underline for emphasis, a note comment for a longer thought in the margin. 3. To highlight text, drag the cursor across the words. The mark sticks to the line and will not slide, even if the file is opened on a phone. 4. For scans and images, use a freehand line, an arrow, or a rectangle. These sit on top of the page and do not need a text layer. 5. Use color to separate meaning. Yellow for questions, red for required edits, for example. That way the recipient does not have to guess what matters more. 6. Save the result and download the new PDF. The annotations stay inside the file and open in any reader.
What can go wrong
- Text will not highlight and the marker will not stick. You are looking at a scan with no text layer. Use freehand markups, or run OCR on it first.
- A margin note shows up as a small icon instead of open text. That is normal: pop-up comments expand on click. If the recipient needs to read them right away, repeat the point with a box or an arrow on the page itself.
- After flattening annotations into the page, the comment cannot be removed. It is printed into the image. Flatten the layer only when the edits are final, otherwise keep annotations editable.
- A highlight color is barely visible on a dark background or a colored block. For spots like that, a box with a contrasting outline works better than a fill.
- Your markup accidentally erased the original text. That means you switched into content editing instead of commenting. Annotations do not touch the text. If you actually need to change words, that is a different operation.
What to check before you send
- Every mark is on the right page and did not shift during scrolling.
- Pop-up notes hold clear text, not empty icons.
- The color logic is obvious without explanation, or you added a short legend.
- The file opens with its annotations in an outside viewer, not only in the editor.
- If the document goes outside the company, no internal remarks are left in the comments.
What this pairs well with
Once the edits are agreed on, the document usually moves further down the line. To add a signature, use Sign PDF, and to enter data into form fields, use PDF Filler. If the file has confidential parts that should not be left under a highlight, cover them with Redact PDF before sending. All of these steps happen in the browser with nothing to install, and uploaded files are deleted automatically after 120 minutes.