The Username field is not case sensitive. The Password field can be set as case sensitive and is controlled under the document settings.
If you use case sensitive password, make sure your User knows the proper capitalization. Users sometimes encounter issues if the username or password contain capital or lowercase Ls and Is and Os (which are often confused with zeroes 0). If you can, you should try to avoid these characters.
Be careful in using spaces for username and password. The system will require the Users to enter the username and password with spaces added even if it is the first or last character in the field.