Complemento di agente = agent in a passive sentence (it is introduced by "by" in English and often by "da" in Italian)
The apple is eaten by Mark.
by Mark = agent
The agent woud be the subject in the active voice: Mark eats the apple
The documents [you filled out] are on the table.
I documenti [che hai compilato] sono sopra il tavolo.
The sentences in brakets are regular, active sentences. Now let's rewrite them in the passive voice.
The documents [that were filled out by you] are on the table.
I documenti [che sono stati compilati da te] sono sopra il tavolo.
We can make it even shorter:
The documents [filled out by you] are on the table.
I documenti [compilati da te] sono sopra il tavolo.
The agent (by you/da te) in this case is a pronoun (te), therefore it sounds better in Italian to move it before the past participle (filled out/compilato)
I documenti [da te compilati] sono sopra il tavolo. = ❌ The documents [by you filled out] are on the table.
Had the agent NOT been a pronoun, but just a noun (for example, "by the policeman/by Carlo", then it could have not been moved before the past participle:
I documenti [compilati dal poliziotto/da Mario] sono sopra il tavolo.
I know it might sound confusing at first but that's just how it works. German, for example, can only have such word order:
Die von dir ausgefüllten Dokumente liegen auf dem Tisch.
"The by you out-filled documents lie on the table"