why does Reader X consume so much CPU time all the while a document is open?

I open a document (plain printed text) in Adobe Reader X (version 10.1.6 running on Mac OS X 10.6.8), and it's consuming 90% CPU or more all the time. This is intolerable if I need to keep the document open to refer to while I'm working. This happens only on the occasional document, not every document; most are well-behaved.

Is this a general problem with Reader X, or is there some plug-in or preference that I can disable to solve this problem, or is there some problem with the document that I can fix?

TIA, Mike