Unable to select and highlight on Preview in OSX Lion
PDFs are just a wrapper for various kinds of content.   Depending on how the PDF was created the text might be selectable.   A lot of them have the text stored as graphics and in that case you wont be able to select/copy/highlight it.   I suspect PDFs also allows the "protection" of content so that viewers cannot select and copy bits of text.

Where did you get the PDFs if you can track the source you can change how they are created.   If it was prom a scan it will normally be a picture.   Utilities like PDFPen etc.  allows OCS of Scans.  If you do that the text will become selectable/highlightable.