The Comment tab in the PDF Editor allows collaborating on a PDF using comments and various markup tools.
The corresponding window of the Online PDF Editor:

The corresponding window of the Desktop PDF Editor:

Using this tab, you can:
- highlight the selected text,
- add comments,
- apply strikeout or underline formatting to the selected text,
- mark up a PDF using one of the three available pens,
- show or hide comments using the corresponding checkbox.