I want altitude flying, as high as peregrine falcons or vultures (so bat-like creatures are discarded because as far as I know they only fly at low altitudes);
Real flying, not gliding;
There's a misconception here about flying.
Most of real flying is gliding.
Any creature flying does as little work as possible when in the air. They glide. They use air currents. Seagulls and other large avians are fine examples.
Gliding, contrary to what you may think, does allow you to rise very high. The world altitude record is, I think, some 15 km (that, 15000 meters).
The main limitation for altitude in normal atmospheres would be that the density of the atmosphere will decrease, making it likely that a creature flying that high would need to build up and store whatever it breathes and could only stay at such heights for relatively short periods.
But gliding is the heart of flying.
Flapping your wings is not a good plan, anymore than a ground creature would survive long running everywhere. It's energy inefficient. It's for getting off the ground (at a run usually) and redirecting your flight with occasional bursts if you absolutely have to stay up.
Not beaks with teeth as some birds on Earth. Actual well defined jaw bones, like most predator mammals or crocodiles;
A beak with teeth is just as effective for a predator as a well defined jaw bone. A lot of razor sharp small teeth backed up by a long hard pointy stabbing object like a beak is a pretty vicious combination.
Part of the beak shape is for aerodynamic effect. A well defined jaw is a heavy object with little practical use for flight control. The more your creature relies on bite (which is implied for such a design), the more muscle they need in the jaw and weight is last thing an avian design needs.
But if you insist in such a thing, go ahead. It's not written in stone that an avian creature has to have a beak.
If you'd ever seen an eagle, those feet-claws are pretty deadly - I saw one attack a large loon in a lake one time and believe me, feathers and a beak are no hindrance to being a predator !
Quite large creature, but not trespassing the limits of weight of flying creatures of our current real world physics;
Like a large glider than can reach 10km in altitude and glide a distance of 1000km ?
Sounds fine to me.
You really need to read up on gliding to develop this idea properly, IMO.
But consider - what advantage or practical evolutionary niche is gliding or flying very high giving your creature ?
Bonus points if the reasons of it being possible fits a scaled animal rather than feathered.
Feathers are useful for control surfaces and insulation (cold at altitude and fat may work for ground creatures, but it's too heavy for avians to use much).
Scales are fine for armor, but they're heavy (have to glide and lift into the air). It's worth noting that your basic avian predator is hardly likely to benefit from scales. They have the ability to limit their exposure to attacks from ground creatures and they can hunt in small groups (like many modern predators). Even a family or clan sized unit would be a dangerous combination for a ground creature.
I think scales are not needed. A reasonably tough skin would be fine.