Highlighting can be performed in the PDF editor either at a document level or bundle level. More guidance on opening documents or bundles in the PDF editor can be found here.
With your document open in the PDF editor, first select the "Pen" tool in the ribbon, and select "Freeform Highlight".
With the freeform highlight tool selected, you can click and drag over your PDF to highlight.
Before highlighting on your document, you can change the highlight colour, line thickness, line opacity and blend mode using the sub-ribbon.
