//Using a UNC share for document location
//User Name can be speficied as Domain\User, User@Domain (UPN format), MachineName\User
//or User (local user). If a value contains semi-colon character, that value should be enclosed
//in single quotes (eg. Password='PASSWORD') or double quotes (eg. Password="PASSWORD").
documentViewer.DocumentLocation = @"Path=\\server\share; User Name=USERNAME; Password=PASSWORD";

//Same as above
documentViewer.DocumentLocation = new PhysicalLocation
{
    Path = @"\\server\share",
    UserName = "USERNAME",
    Password = "PASSWORD"
};