With the PDF Import premium feature you can import a PDF into Squid as a new note and easily write, draw, and markup on the PDF just like a standard note. A PDF is displayed as the background of a note in the original PDF format, so zooming in re-renders the PDF (as well as all your handwritten notes) in high quality. If you want to share your marked up PDF note with others, simply export it as a new PDF or set of images. Exporting a PDF note as a new PDF preserves the original PDF while adding all your handwriting and markups to it!
The highlighter tool is also included in the PDF Import premium feature.