Highlight PDF

A highlighter that follows the text

Dragging a rectangle over a paragraph gives you a coloured box that covers the margins, the gaps between lines and half of the line below. That is not what a highlighter does. When you drag across text here, the editor finds the lines of text your drag touched and highlights each of them individually, from where the text starts to where it ends — or, on a single line, just the stretch you dragged across. The result looks like it was done with a pen, because it follows the same shapes a pen would.

Highlights are saved with a multiply blend, the way ink behaves on paper: the colour darkens what is under it rather than painting over it, so black text stays black and fully legible underneath. That is also why highlighted text remains selectable and searchable — nothing about the text itself is changed.

Yellow is the default because it is the colour readers expect, but any colour works, and the strength can be adjusted. A consistent scheme helps when a document is reviewed by several people or read again months later: yellow for key points, green for definitions, pink for things to check, for example.

The rest of a markup kit

Most marking-up is a combination of a few things. Arrows point at a figure or a clause; rectangles and ellipses circle what matters; a short typed note says why. All of them are in the same toolbar: draw an arrow by dragging from tail to tip, hold Shift to keep it straight, and type a note next to it with Add text. The Draw tool gives you a freehand pen for ticks, crosses, underlines and handwriting, and it works with a stylus or a finger on a tablet or phone.

Everything stays editable until you save. Select an annotation to recolour it, change its line width or opacity, move it, resize it, duplicate it with Ctrl+D or delete it, and use undo freely. Your last-used colour and width carry over to the next mark, so a series of annotations comes out consistent without any fiddling.

Annotations that every reader shows the same way

PDF has a separate annotation layer — comments and markup stored alongside the page — and different readers display it differently: some hide it when printing, some show comment icons instead of the markup, some mobile viewers ignore it altogether. Markup made here is drawn into the page itself, so it looks the same in every reader, in a browser, on a phone and on paper. The trade-off is that it does not appear in a reader's comment list and cannot be toggled off; for reviews where that matters, keep an unmarked copy alongside the marked one.

Because everything is drawn as vectors, highlights and shapes stay sharp at any zoom and add very little to the file size — a heavily annotated page typically grows by a few kilobytes.

Who this is for

Students marking up lecture notes and papers, teachers correcting submissions, lawyers and paralegals flagging clauses, reviewers commenting on a draft, anyone preparing a document for a meeting. The common thread is that these documents are often not yours to upload: exam scripts, client contracts, internal reports. Nothing here is uploaded — the PDF is opened and saved entirely in your browser.