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".

PDF files are a poor format for text extraction, and FilingDB recommends that you check for data in other formats before extracting text from PDF files.