Can creatures attack anything within their natural reach?

Question

I get the sense that there is a difference between a natural reach of 10ft and a weapon reach of 10ft, namely that you should be able to attack anything within your natural reach, but I can't find anything clear-cut about it.

For weapons, it's made very clear that if they allow you to threaten and attack something 10ft away instead of 5ft, the downside is that you can't threaten or attack creatures adjacent to you.

With a typical reach weapon, you can strike opponents 10 feet away, but you can't strike adjacent foes (those within 5 feet).

Reach Weapons: Most creatures of Medium or smaller size have a reach of only 5 feet. This means that they can make melee attacks only against creatures up to 5 feet (1 square) away. However, Small and Medium creatures wielding reach weapons threaten more squares than a typical creature. In addition, most creatures larger than Medium have a natural reach of 10 feet or more.

Reach Weapons: (...) Most reach weapons double the wielder's natural reach, meaning that a typical Small or Medium wielder of such a weapon can attack a creature 10 feet away, but not a creature in an adjacent square.

But what about natural reach greater than 5ft? Large creature tend to get such a reach. Large tall creatures get a natural reach of 10ft.

Creatures that take up more than 1 square typically have a natural reach of 10 feet or more, meaning that they can reach targets even if they aren't in adjacent squares.

Unlike when someone uses a reach weapon, a creature with greater than normal natural reach (more than 5 feet) still threatens squares adjacent to it.

It threatens... but can it attack? I guess the question here is: is a threatened square a square you can always attack into?

Then there's the case of large creatures using a reach weapon:

Large or larger creatures using reach weapons can strike up to double their natural reach but can't strike at their natural reach or less.

A typical Large character wielding a reach weapon of the appropriate size can attack a creature 15 or 20 feet away, but not adjacent creatures or creatures up to 10 feet away.

This could imply that usually they can "strike at their natural reach or less" I suppose.

This actually seems simpler: reach weapon = can't attack within natural reach but can attack beyond and up to double natural reach.

Answer 1

Yes, creatures can attack anything within their natural reach

As you pointed out,

Creatures that take up more than 1 square typically have a natural reach of 10 feet or more, meaning that they can reach targets even if they aren't in adjacent squares.

Unlike when someone uses a reach weapon, a creature with greater than normal natural reach (more than 5 feet) still threatens squares adjacent to it.

So what does "threaten" mean?

You threaten all squares into which you can make a melee attack, even when it is not your turn. Generally, that means everything in all squares adjacent to your space (including diagonally).

As a visual illustration that natural reach doesn't exclude adjacent squares (or just to visualize reach in general, especially diagonal reach) these graphics can be helpful as well.

Answer 2

A reach of 10 ft means the ability to attack anything within 10 ft, however gained. Reach weapons are an exception, unable to attack adjacent foes.

Answer 3

The rule you need is here:

Large, Huge, Gargantuan, and Colossal Creatures

Very large creatures take up more than 1 square.

Creatures that take up more than 1 square typically have a natural reach of 10 feet or more, meaning that they can reach targets even if they aren't in adjacent squares.

Unlike when someone uses a reach weapon, a creature with greater than normal natural reach (more than 5 feet) still threatens squares adjacent to it. A creature with greater than normal natural reach usually gets an attack of opportunity against you if you approach it, because you must enter and move within the range of its reach before you can attack it. This attack of opportunity is not provoked if you take a 5-foot step.

Large or larger creatures using reach weapons can strike up to double their natural reach but can't strike at their natural reach or less.

And here:

You threaten all squares into which you can make a melee attack, even when it is not your turn. Generally, that means everything in all squares adjacent to your space (including diagonally). An enemy that takes certain actions while in a threatened square provokes an attack of opportunity from you. If you're unarmed, you don't normally threaten any squares and thus can't make attacks of opportunity.

(emphases mine)

Generally speaking, reaches and ranges typically include everything out to its furthest extent; reach weapons are the exception, though they are the more common case for PC reach.