What's the difference between an adjective and a past participle? [duplicate]

I'm really confused about the object

the doctor specialized will help you

or

the documents required

How can I tell in the future, if I have to use the past participle or the adjective? And what's the name of this structure?

There is no "difference" in the sense that a word i necessarily one or the other.

The participle (present and past) can be used as an adjective in many cases, in exactly the same way other adjectives would be used.

What the teacher said, has interested the student.
The student was interested in what the teacher was saying.
The interested student was listening to the teacher.

In the first sentence we see interested as a participle, and that same participle is used in the third sentence as an adjective. In the second sentence, it can be read as either the participle or as an adjective.

We can do the same with the present participle:

The student is talking while the teacher speaks.
The teacher tells the talking student to keep quiet.

The basic forms of the verb to break are break broke broken. The third form is called past participle. The Latin term participle means something that takes part (in two things) and the third verb form is called participle because it is a verb form that can be used as an adjective as in - a broken window/ a broken man/ The window is broken or as a form with verbal character as in- The window broken by the school boys must be repaired. "broken by the school boys" is a shortened relative clause: - The window that was broken by the school boys ...

So, from the way how a past participle is used in a sentence you can see whether it is used as an adjective or a participle.