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.

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.

This post helps to add a text box to PDF using a free software and an online tool. You can adjust the size of text box, text style, etc.

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 ...

Adding Text Notes to a PDF Select the Text Box tool from the Drawing Markups panel. Click, or click and drag, on your document to add a box with a colored border, and type your observations in it.

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.

Need to add text to a PDF that doesn't support editing? No problem -- and no extra software required.