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The marks this tool adds are annotations, not drawings. A real highlight appears in a reader's comment list, carries your name, can be replied to, filtered and deleted; a yellow rectangle painted onto the page is ink for ever. Text markup snaps to the words underneath rather than to a rectangle you drew approximately over them.

Use it when a document is going back to whoever wrote it: marking the clause you disagree with, striking a sentence that should go, leaving a note beside a figure that looks wrong, or circling the part of a drawing that needs redoing.

วิธีทำงาน

1. Drop the PDF onto this page and wait for it to render.
2. Select words to highlight, underline, strike out or squiggly-underline them, or pick a shape, a note or the pen for freehand ink.
3. Set the colour and opacity, and type the comment that goes with the mark if it needs one.
4. Put your name in Author so the recipient's comment panel says who wrote what.
5. Leave Turn the comments into ink off unless the marks must not be editable.
6. Press Annotate and the download starts.

Text markup follows the text. The page's text runs are read first, so a sentence that wraps over two lines becomes one annotation with a quadrilateral for each line, not one wide box slanting across the paragraph. Those quadrilaterals are written in the order the specification asks for - upper-left, upper-right, lower-left, lower-right, which is neither clockwise nor anticlockwise - and getting it wrong is why highlights from some tools render as bow-ties in Acrobat.

Nine kinds of mark are available: highlight, underline, strikeout, squiggly underline, sticky note, square, circle, line and freehand ink. All are written with the print flag set, because a review comment that vanishes when the recipient prints the document is a comment they never read. Highlights default to 40 percent opacity so the words under them stay legible.

No appearance stream is written for these annotations, so each reader draws them from the annotation data using its own conventions: a highlight looks like an Acrobat highlight in Acrobat and like a Preview highlight in Preview, and the exact edge and shade differ slightly. In exchange, the marks stay editable data rather than becoming pixels.

Turning the comments into ink runs the same pass as Flatten PDF: each annotation's appearance stream is drawn onto the page and the annotation deleted. Only marks that carry such a stream can be drawn, and ones the reader synthesises do not, so flattening immediately after adding leaves them in place. Open the file once in a reader and save it - that writes the appearances - then flatten.

Nothing about the document is sent anywhere: the name you put in Author is written into the file you download and nowhere else.