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. Edit texts with format ...

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.

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.