Some Acrobat PDF documents cannot be edited or marked with the Acrobat Pro fluorescent marker (Highlight Text Tool). This tool is extremely useful to mark up text for future reference, teaching, research, etc., but it seemingly can't be used on these documents. For example, any of the PDFs found here, such as this one [312KB PDF].
The solution turns out to be fairly simple. Open Acrobat Pro's preferences, and select the Documents section in the left-hand column. In the PDF/A View Mode section on the right, set the pop-up menu to Never, and click OK. From that moment on, all 'PDF/A' documents can be highlighted with Acrobat. Great!\
[robg adds: This fix works as described.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20100226065654996