Annotate a PDF with Highlights and Notes
Annotating a PDF adds visual remarks — translucent highlight boxes and short text notes — on top of the existing pages so a reviewer can show what matters and why. This tool renders each page as a live preview in your browser, lets you draw highlights and place notes exactly where you want them, then flattens those marks into the page content and saves the result, all without uploading the document anywhere. The existing text is never changed or deleted; annotations sit over it.
How it works
The tool combines two engines that never talk to a server. The first is a PDF renderer (pdf.js, the same engine inside Firefox), which parses your document and rasterizes each page onto a canvas so you can see exactly what you are marking up. As you drag out a highlight or click to place a note, the marks are recorded not as pixels but as geometry: a rectangle's corners, a note's anchor point and text, all in the preview's coordinate space. That distinction matters, because the preview is just a picture — the real work happens at save time against the original file.
When you save, each recorded mark is converted from preview coordinates to PDF page coordinates. The two systems disagree in a way that must be handled explicitly: canvases count y downward from the top-left, while PDF measures y upward from the bottom-left, so every rectangle is flipped through y_pdf = pageHeight − y_canvas − height and scaled by the ratio between preview pixels and page points. The converted marks are then written into each page's content stream: highlights become filled rectangles painted through a transparency graphics state (typically yellow at around 40% fill opacity, so the text beneath stays legible), and notes become small text runs in an embedded standard font.
This is flattening: the marks end up as ordinary page content, not as PDF annotation objects of the kind Acrobat's comment tools create. The difference cuts both ways. Annotation objects live in a separate layer — viewers can list them, hide them, and other people can edit or strip them, and some viewers don't print them by default. Flattened marks are unconditional: every viewer renders them, every printer prints them, nobody can switch them off — and equally, you cannot un-highlight later, which is why the tool always leaves your original file untouched. One honest limit follows from drawing on top: a highlight over private data is not redaction. The characters remain extractable underneath; genuinely removing them requires a redaction tool that rebuilds the page.
Worked example: reviewing a 22-page lease
A tenant reviews lease-draft.pdf, 22 pages, 1.1 MB, before a call with their lawyer. Across pages 4, 9, 11, and 17 they draw six yellow highlight boxes over clauses about break fees and maintenance duties, and place four notes such as "check against council schedule" beside them. Rendering the preview takes about two seconds for the full document; drawing is instant since it happens on canvas. Saving flattens ten marks and produces a 1.13 MB file — the additions are a few hundred bytes of rectangle operators, one graphics state, and a shared font. Opened afterwards on an iPhone's stock viewer and printed from a library PC, all ten marks appear identically, which annotation-object comments from some apps would not have guaranteed.
Frequently asked questions
Are these annotations removable comments or permanent marks?
Permanent marks. The highlights and notes are flattened into the page content when you save, so every viewer shows them and no viewer can toggle them off. Keep your original file if you want an unmarked copy.
Can I edit or delete the PDF's existing text while annotating?
No. Annotation draws on top of the page; it does not reflow, rewrite, or remove what is already there. To cover content cosmetically you can place an opaque box, but the text underneath still exists in the file.
Will my highlights show up in other PDF viewers and in print?
Yes. Because the marks become ordinary page content rather than annotation objects, they render identically in Adobe Reader, browser viewers, and mobile apps, and they always print.
Is a flattened highlight suitable for redacting private data?
No. A highlight or even a solid box only draws over the text; the characters remain in the file and can be copied out or extracted. Use a true redaction tool that removes the underlying text.
Does annotating here upload my document to a server?
No. The preview is rendered locally, your marks exist only in browser memory, and the flattened PDF is generated and downloaded on your device. The document never crosses the network.
Why does my scanned PDF highlight fine but not let me select text?
A scanned page is one big image with no text layer, and this tool draws highlight boxes by position, not by selecting characters. Highlighting works fine visually; text selection would require OCR, which this tool does not perform.