Are ubuntu software center sources different from apt-get / synaptic?

I'm trying to download master-pdf-editor package from official repositories. All sites report that it's easy to do: just download it from ubuntu software center. I can't install the software center, so I'm trying to install it via apt-get or synaptic, but I can't find the package in the repos...

Can anyone help?

Some packages in the Software Center may be free but are offered under a proprietary license. For this reason (the license) they are not included by default in the Ubuntu apt repositories.

When you install one of these packages via the Software Center it will create a new apt source file in /etc/apt/sources.list.d/. In the case of Master PDF Editor it will create:

private-ppa.launchpad.net_commercial-ppa-uploaders_master-pdf-editor_ubuntu.list

Once that file has been placed in /etc/apt/sources.list.d/ you can then use the regular apt/aptitude commands to update it.

I also want to add that sources like this often require authentication. If they do a comment will be placed in the source file indicating as such:

$ cat private-ppa.launchpad.net_commercial-ppa-uploaders_master-pdf-editor_ubuntu.list 
deb https://private-ppa.launchpad.net/commercial-ppa-uploaders/master-pdf-editor/ubuntu precise main #Added by software-center; credentials stored in /etc/apt/auth.conf

At the end you can see the comment, "credentials stored in /etc/apt/auth.conf". That file will store a hash of your password like so:

$ sudo cat /etc/apt/auth.conf # Needs root to read the file
machine private-ppa.launchpad.net/commercial-ppa-uploaders/master-pdf-editor/ubuntu
login riskable
password someGobbldyGookHash

No, they're the same.

For purchased things that you can't install without the Software Centre, it actually just adds a new (authenticated) repository for that application... But it's all going through apt and once that repo is added you can use the standard apt-get/dpkg toolset.

So in this case (With master-pdf-editor) you need to go through Software Centre (for it to add its commercial repo).