Speaking of manuals, the PSR-S670 has three.
Start with the Owner's Manual, which should be a relatively brief run-through of the features and how to use them.
The Reference Manual is a more in-depth description of how to use the features, so turn to it after you've looked at the Owner's Manual but want to learn about a particular topic in greater detail.
The Data List contains lists of your keyboard's built-in voices, styles, effects, etc., as well as information about the MIDI messages it can use. Most of that content-- with the exception of the MIDI information-- should be accessible through your keyboard's menus, so you shouldn't need to refer to the Data List unless you want to see a particular list "all at once" rather than having to scroll through menu screens, or need to look up a MIDI message.
My advice is to stick with the basic operations first, focusing on just those features you're most interested in, and save the other features for later when-- or if-- you start to take an interest in them. Just because a feature is there doesn't mean you need to use it. But it might still be a good idea to at least glance at each page of the Owner's Manual so you know what features are available.
Also, I recommend that you go to the Download section of Support at Yamaha's web site and download the PDF versions of the manuals, because it's extremely handy to be able to search in the PDF files for a particular word or phrase.
