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If you're in ProQuest Ebook Central:
Along the top of the ebook is a toolbar with annotation options: highlighter, add notes, bookmark.
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Highlighter: Highlight text in different colors using the icon at the top
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Or highlight text using the cursor, and look for the highlighter tool to appear.
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Video demo:
Saving highlighted sections: Your highlighted sections will be automatically saved in the Annotations section of the ebook, and linked to your account.
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In Accessibility Mode
When the accessibility mode is turned on, highlighting is not available.