I’ve always been puzzled by the use of the expression "ci-joint" (enclosed, as in "please find enclosed…") in French; does it need to agree with the noun it is linked to or not? Checking again today, I realised that I’ve seen it used in different ways because its use isn’t fixed when it is in the middle of a sentence.
The general rule is:
It is invariable when it has an adverbial value :

  • at the beginning of a sentence : ci-joint les factures… (please find enclosed the bills…)
  • in the middle of a sentence, immediately followed by a noun : vous trouverez ci-joint factures…

It agrees with the noun when:

  • it is placed straight after the noun and has the value of an attribute: les factures ci-jointes…
  • it is an attribute of the subject : les factures sont ci-jointes…

In all other cases, it is up to the writer to decide whether it should agree or not. The Académie française website gives interesting examples of the stylistic choices made by famous French authors :
Bernanos : "Vous trouverez ci-joint les pages dactylographiées de mon roman" (Please find enclosed the typed pages of my novel) NO AGREEMENT
Hugo : "Je vous envoie ci-incluses des paroles prononcées ici par moi au moment de la proscription" (please find enclosed the words I pronounced during the ban) AGREEMENT
Musset : "Je prends la liberté de vous envoyer ci-jointes des rillettes" (Please find enclosed some rillettes, which I’ve taken the liberty to send you) AGREEMENT