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.