If you want to extract highlighted text from PDF file as a plain text file, then this post covers some good free PDF Highlight Extractor software to do that.

Using the PDF Tool & Inking Feature in Microsoft Edge, you can draw on PDF files, highlight text, add text, and choose color. Learn how!

ReaddleDocs’s PDF viewer now lets you highlight text inside PDF files in several colors, make notes at any place in the document, and save these changes in a desktop-compatible PDF format.