Some PDFs won't allow me to highlight or make notes...

[mabw97]

When I try to make highlights or notes on articles from academic journals, some of them will easily allow me to highlight or make notes. Others won't let me do this at all. I am wondering if it's because some PDFs are converted from Word files while others are scanned in and are in essence a picture. It's not that it's some journals but not others, for example Research in the Teaching of English has some articles that allow me to highlight and annotate, while other articles from that same journal only give me the option to "copy."

Is there any app, add-on, or program that will allow me to highlight and take notes on the stubborn journal articles? Any other advice on how to solve this?

[Claudio González]

Try selecrting a couple of words in the "stubborn" files. If you can't, they are images of text, and you would need to convert them to text first. Also, some authors take measures for preventing their PDF files to be copied or printed.

[pwillener]

Highlighting will only work on text documents, not scanned images.  From another recent post by ~graffiti:

What I've recommended to people in the past is to use the drawing markup tools to draw boxes around the text they want highlighted then play around with the fill color of the box until they get something close to what they want.

Commenting may be disabled for a document; download such a PDF to your local disk and check its properties:

[alexa97681874]

If you go to the "Recognize Text" drop down and select "In This File" it will recognize all the texts in the document. After that, you should be able to highlight.

[try67]

That only works in Acrobat, not in the free Reader...