To change the default magnification that Acrobat or Acrobat Reader will use when displaying a PDF make the following change while there are no documents open in the program.
Go into Preferences in the File menu and then select General. This will open the General Preferences dialog box.
The second box in this window is labelled Magnification and these are the fields that you need to change to alter this default. The main field to select your new default in is the Default Zoom field where you can choose either a fixed percentage of full size at which all pages will be displayed or alternatively choose one of the following options:
Once you have selected your default magnification you can press the OK button to apply your change. Before you do that you might also want to consider making any changes to the other defaults that appear on the General Preferences screen.
This article written by Stephen Chapman, Felgall Pty Ltd.