Turning off passwords is a bad idea. It gives anyone who accesses your machine complete administrative access. I would be skeptical of any advice in that PDF file. As a new user, it might be best to do some research before following advice that results in significant system changes.
Also, it is best to edit the sudoers file with
visudo, which is a
special editor just for the purpose.
You could try this to whether you can undo the change to sudoers:
sudo visudo. Failing that, you could try
but that is a complete shot in the dark.
Why this would affect your ability to download files is puzzling; it might help to know what the downloads were. To look for them, try opening up the file manager (I forget which one Bohdi uses) and looking in your home directory; perhaps there is a subdirectory named "Downloads." If you downloaded them with Firefox, that application likes to default to putting downloads in a Downloads directory.
