Need a PDF reader which can keep portrait form.

Hello, I am new iPad user. And I have a problem. I read a lots of PDF book. And I love to read them before I go to sleep. Right now I am using Adobe pdf reader. It has a problem. when I turn my side it start rotating. I want to keep it steady.
But I can't find any option to restrain it. Dose I need another PDF reader? Or there is any setting that I need to change. Please help me. Thanks in advance.

thanks for your solution, it works