What a redline is for
A redline is a reviewer's instruction drawn on the sheet: cloud what has to change, point at it, and say what is wrong. Plan reviewers redline submittals, engineers redline their own checksets, and contractors redline field changes for the as-builts. The tools are the same everywhere, and so is the test of a good redline: whoever picks up the sheet next can find every comment, see exactly what it refers to, and act on it without a phone call.
How to redline a PDF drawing online
Cloud the change
Choose Cloud and click two opposite corners around the area. The scalloped border is the convention every reviewer recognizes for revised or questioned work, and it stays clear of the linework underneath.
Point at it and say why
Choose Callout, click the thing the comment is about, click where the note should sit, and type it. On a vector PDF the arrowhead snaps to the nearest endpoint, intersection, or line, so "Revise invert" points at the structure, not near it. Use Arrow for a pointer without a note, and Text for general notes in the margin.
Check, then download
The list beside the sheet holds every markup on it. Select one to recolor or delete it, undo anything, and move through the set sheet by sheet. Download writes all of them into the PDF at once.
Colors that mean something
Red is the reviewer's color by long habit, which is why it is the default. Many offices add a second color for a second reviewer or a different kind of comment: blue for questions, green for items resolved or accepted. Pick a color before you place a markup, or select an existing one and pick a new color to change it.
Redlines that survive the round trip
A flattened redline is part of the page, so it looks the same in every PDF viewer, prints the same, and can't be hidden by a viewer setting. The rest of the file is untouched: the original linework stays vector, PDF layers keep working, and sheets saved sideways keep their orientation with the markups where you put them.
Good redline habits
- One comment per callout, written as an instruction: "Add 6" curb per detail 3/C-500", not "curb?".
- Cloud the whole area that changes, not just the spot that caught your eye.
- Put general comments in a text note near the title block, so they aren't mistaken for one location.
- Keep the redlined PDF with the review letter, so the two can be read side by side.