The Typewriter tool in Acrobat or Adobe Reader 8 is deactivated after each text entry that you make in a PDF file. You have to select the Typewriter tool for each text entry you make.
In Acrobat and Reader 7, the Typewriter tool remains active until you manually deactivate it, or select another tool.
Update to Acrobat or Reader 9. The typewriter tool has been fixed and will remain active.