The 790 went through 3 revisions during it's production run. There were engine changes as well as wiring/electrical system changes.
I suggest you get the serial number (it's on a plate just above the rear PTO shaft) to ensure you get the correct manual.
The CD-ROM version of the manual isn't that expensive (about $50-$60, IIRC) and it's a worthwhile purchase if you intend to keep the tractor for a while.