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.

Sometimes you need to quickly remove a few pages from a PDF, or merge it with another document—jobs that aren't worth forking out for an Adobe Acrobat subscription for. PDF Barber is a free website ...

The PDF format lets you share a file with other people that preserves its exact appearance at the time of creation. But you may not always want people to have the ability to manipulate the file.