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.

There's nothing worse than opening a PDF and realizing you can't use the search function or even highlight text. This typically happens when a PDF was created by scanning a paper document—it's ...

In other words, Adobe Reader won't let you add text. What you need is a program that's a little more accommodating, PDF-wise. Try Foxit Reader 4.3, which allows you to add text to any PDF.

Need to edit a PDF document but don’t want to pay for the more expensive Adobe editors? You can add text to and sign a PDF with just the free Reader program.

PDF Replacer is a free software to find and replace text in bulk in PDF files. You can choose font types, remove text from PDF files using blank,etc.