Did you know PDF files can specify initial layout and zoom settings that take effect when you open them? (For example, when you choose the Export to PDF command in OpenOffice, a dialog box lets you specify the initial view and other user interface options that are saved in the PDF file.)
Your problem may be caused by certain PDF files changing the view settings when you open them. In that case, Adobe Reader has accessibility options to ignore the settings specified in PDF files.
I don't have Adobe Reader 9.1.1. But in Adobe Reader 8.2.0, I was able to go to Edit, Preferences, Accessibility, and there's a check box "Always use Page Layout Style" with a drop down box where you can choose "Single Page Continuous".