Hello GeneDeJ,
Thank you for posting to Apple Support Communities.
If you’re experiencing issues with highlighting a PDF, these steps can help:
1. In the Preview app
on your Mac, open the PDF you want to change.
2. Do any of the following:
• Quickly highlight, underline, or strike through text: Select text, click the down arrow next to the Highlight button
, then choose a highlight color, underline, or strikethrough.
• Use highlight mode: Click the Highlight button
(so it turns gray), click the down arrow next to the Highlight button, then choose a highlight color, underline, or strikethrough. Now, whenever you select some text, the highlighting style you chose is automatically applied to the text.
To turn off highlighting, click the highlight tool again.
• Remove a highlight, an underline, or a strikethrough: Control-click the text, then choose Remove item from the shortcut menu.
This information is provided in Highlight, underline, and strike out text in a PDF using Preview on Mac.
If those steps don’t help, we’ll need some additional information to provide more specific support.
Do you experience this with all PDFs you attempt to highlight?
Did you recently make any changes to your Mac prior to this issue occurring?
Cheers.