How the creator of a PDF file sets up her document determines the extent to which you can use Acrobat Reader's commenting tools to mark the file with notes about its content.

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.

To see alt text in a completed PDF document, you'll need to use a screen reading app, which is an accessibility tool for visually impaired users.

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.

You can type on a PDF using one of several different options, depending on the device you are using.

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.

That PDF file is usually the final stop for your project and doesn’t allow for simple editing after it has been exported, unlike a text document or Google Docs file.