You can easily highlight a PDF on your Mac computer using the markup tools in the built-in Preview program.

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

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.

Removing text from a PDF isn't as easy as drawing a black bar across it. Fortunately, redaction tools let you truly erase sensitive text in Preview, Adobe Acrobat DC Pro, and PDFPen.

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.