I thought it was because my pdf files were big. Then I realized that it is probably not the case, even with smaller pdf files, Preview seems to be very choppy, which make reading PDF a nuisance. Am I the only one with this problem? I haven't tried other PDF reader applications, if you have this problem too, kindly share with me how you get around it. I am using a late 2014 iMac 5K with 16GB ram and the maxed up graphic card, also have the latest 10.11.1 installed.