Need help understanding highlighting in Reader

I have a couple issues with highlighting in Reader.  

First, I have downloaded a publication from the IRS and I wanted to highlight certain sections prior to sending out to employees:  unfortunately, highlighting doesn't seem to work on this PDF document.  Why?

Additionally, I have another document also downloaded from the IRS, but when I try to highlight a sentence or example, the area highlighted is NOT just the section I highlighted, but everything from that section/sentence to the top of the page.  It won't seem to let me highlight just a sentence (or word, or paragraph, etc.) at a time.  Why?

Is there some difference in the way Adobe treats PDF documents, and if so, how can I tell what behavior to expect?  I really don't want to have to print the document and manually highlight it and then scan it back in ... that seems to be a real stupid way to spend time because Adobe isn't consistent in highlighting PDF documents. I will appreciate any suggestions, or even just explanations, and especially any workarounds.