I am trying with both a PDF and/or a PNG file, but this does not seem possible. Nothing I am trying works. Is it possible to take a screenshot natively in macOS, then highlight a portion of said screenshot? If so, how can this be done exactly?
To be clear, I want to just use native tools that are built-in to macOS, so Preview in this case.
In a word, no.
While you can annotate anything in Preview.app it does not offer the option (that I have seen) to use translucent lines or shapes. You can change the color to anything your heart desires of both the line, shapes and shape fill but you can't make it transparent.
Not that this applies only to bitmaps like JPG & PNG where you are taking a picture of the screen or downloading a picture of text from somewhere. If you have a PDF that has actual text in it. EG a PDF that you can select and copy text in then, yes, you can highlight portions of the text:
The difference being "a picture of text: no" or "actual text in a PDF: yes."
If you want to "annotate" a screen capture of text you will need to find a program that lets you overlay a transparent colored shape over another picture. I would bet there are a number of shareware/freeware programs (GIMP springs to mind) that would let you do this.
While Preview's highlighter tool does not work, as a workaround you can create a yellow, partially transparent rectangle over the text and then duplicate that and stretch it to highlight the other desired lines.
The process isn't as nice, but when outputting an image the result is the same.
Actually, if you go to the original source text you hope to "capture" (say a website for example) and then if you "print" the text as a pdf (as opposed to simply doing a screen grab) you can then highlight the text!
Hope this helps!
It's possible now for images because Preview can recognize text there.
But there is a trick - you first need to export your image as a pdf file and then you Tools => Annotate => Highlight Text
