I have observed this increase in cpu usage since the ubuntu 8.10, when I started using acroread.
I thought it gonna improve in the near future, but it still exists, and I am not able to find someone who has seen or reported this.
Currently I am using Ubuntu 11.10 64 bit and adobe reader's version is 9.4.7.
Try reproducing it, just by opening any text pdf document and selecting some text, dont release the left click, keep it holding and it eats full cpu, until you leave it.
System Load Indicator shows a sudden and full increase in usage.
Any one having any idea about it. You cannot report it as Ubuntu bug because adobe is external software.
This is not observed in the web pdf document downloaded and opened in web browser using same adobe's reader.