When you open your PDF document, do you get the dialog like this?
Is the dialog title "Enter Password"? Does the dialog message contain the line "Please enter a Document Open Password"?
If they are different, you have other type of "protected" PDF document that Adobe Reader does not support. Unless you know the correct password for the PDF document, you will not be able to open the PDF document or remove the protection from it.
Could you try opening the same PDF document in the desktop version of Adobe Reader on your Windows or Mac computer and see if it works?