Dear developer,
would you be so kind and make it possible to highlight texts in imported PDFs with the highlighter tool?
That would be kind and helpful!
This is available wherever the PDF toolkit allows for it. At the moment that is solely the Mac, though it looks possible that may be coming to iOS as Apple has finally added the capability to their PDF viewer there.
Refer to §8.1.3, under subheading, Viewing PDFS, in the Mac version of the manual (I see you are cross-platform). There is a yellow call-out box that points out what annotation tools are available.