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.

EaseUS PDF Editor offers a complete set of PDF-editing tools to add edits to PDF files on your Mac quickly and easily, and it even converts scanned documents to text.

It's much easier to add text to a PDF than you may think, and it can be done on any device. Geber86/Getty Images ...

That said, free users can make certain changes to PDF files without a paid plan. As a free user, you can add comments, highlight sections, draw on a document, add new text, and sign documents.

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

Add signatures, text and annotations to a PDF on your iPhone. Type and add text onto a PDF on your iPhone for free.

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.