How it works is that 16 is the total internal storage, but a chunk is used by the operating system and other system partitions and so you only have something like 10-12 of that. And then there's the old binary Vs decimal thing, which I'll explain if you really want to know.
As for moving apps, there are two ways but both require OS support. You can go into Settings > Apps (or is it "App Manager" on a Samsung?), select the app and look for a "move to SD" button. That will move part of the app to SD, but not all apps support it. Or if your OS has the option you can format the card as internal (which will also mean that you can't remove it and use it in a different device as it will be encrypted), which will let you move more stuff. You can't just move apps using a file explorer, and they wouldn't work if you did.
Photos and PDFs, on the other hand, you can just move to wherever you like using a file explorer.