The accuracy of our highlighting tool can depend on PDF quality, so you may have some problems with scanned/poorly rendered PDFs. You may also need to annotate non-text elements of a PDF - we have you covered in both cases! There are PDF drawing tools you can use which will give you various options for shapes, lines arrows, freehand text etc. If you open a PDF in Lean Library Workspace you'll see this icon near the top right of the page, just click it to open the options: