Hi David,
A .PPF file is potentially much more useful to you than a .PPI file, but you must use YEM to take advantage of it!
A .PPI file is what can be installed directly to your keyboard, however only one .PPI file can be installed at any one time, even if it only uses a small amount of the available expansion memory. So if you have a pack which has only been supplied as a .PPI file, you can only install this one complete pack to the keyboard.
On the other hand, a .PPF file can be imported into YEM (use the 'Create Pack or Import Content' options), and then the voices or other content from the pack that you want to install on you keyboard use can be selected, and combined with content from other packs into a single PPI file (up to the available memory of approx. 160 MB on an S770).
So in many cases users will use YEM to select voices from the imported .PPF packs to then combine into a single .PPI file to make full use of the expansion memory space . This .PPI file can then be installed to the keyboard.
Note that some commercial packs are now protected (.CPF) and there may be restrictions about what you can do with these!
Regards
Ian