With a PDF open, click the ‘Highlight Tool’ (paper and pencil icon) in the bottom toolbar. Click and drag over the area you wish to comment on and a resizeable yellow box will appear. Once you've highlighted the appropriate area, start typing to leave a comment. Click "Save" to submit the comment and create an associated issue. Using the options menu (three dots on the newly created issue card) you can open the issue in the left side panel and add more information. For more instructions on this, see “Add Details to Issue”.