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.

In addition, AlterPDF allows you to extract only text from PDF files. Select "Extract Text", specify the page to extract, and click "Extract".

A PDF file is a data format that can be viewed on a PC in any environment without breaking the display of text and images. However, if you try to copy text data from PDF, you may not be able to ...