You may use OCR software to convert scanned PDFs into searchable and editable PDFs. This software works by recognizing text in the PDF and converting it into editable text. Btw, you may give https://smartengines.com/ a try. I'm pleased of their tool. Another option is to use a PDF editor to add highlights and notes directly to the PDF. Finally, you could consider using a different citation management tool that has the ability to add highlights and notes to PDFs. It may be worth exploring other options to see if there is a better fit for your needs.
LeessWeeneys
You may use OCR software to convert scanned PDFs into searchable and editable PDFs. This software works by recognizing text in the PDF and converting it into editable text. Btw, you may give https://smartengines.com/ a try. I'm pleased of their tool. Another option is to use a PDF editor to add highlights and notes directly to the PDF. Finally, you could consider using a different citation management tool that has the ability to add highlights and notes to PDFs. It may be worth exploring other options to see if there is a better fit for your needs.