If you are not able to highlight and annotate .pdf's within RefWorks, this is an indication that it has not been properly OCRd. OCR, which stands for Optical Character Recognition, enables your text to be read by the computer. Without going through this process, your text cannot be highlighted within RefWorks.
The directions below can help you work through this issue.
1. Visit https://ocr.space/.
2. Add your document under "Upload Image or PDF File".
3. Ensure that the box is checked beside "Auto-enlarge content".
4. Under Create Searchable PDF, select “Create searchable PDF with invisible text layer.”
5. Under Select OCR Engine to Use, select “Use OCR Engine1.”
6. Select “Start OCR!”
7. After the process is complete, you will select the “Download Searchable PDF” link.
8. Download the PDF.
9. Upload this new OCRd PDF to your RefWorks record.
After going through this process, you will be able to annotate your PDF within RefWorks.